METHOD OF DEPOSITION OF A THIN SULFIDE LAYER OF A TRANSITION METAL OR OF ONE OF ITS ALLOYS

Abstract

A method of the vapor-phase deposition of a sulfide layer of a transition metal by ALD according to the following cycle: exposing a substrate to a precursor of the transition metal, whereby an intermediate layer is formed, purging the reactor, exposing the intermediate layer to a precursor of sulfur, purging the reactor, the substrate being at a temperature in the range from 20 C. to 250 C. during the cycle, where the cycle can be repeated several times with the same precursors or with different precursors, the precursor of the transition metal being selected from among molybdenum oxyhalides, tungsten oxyhalides, vanadium halides, niobium halides, and tantalum halides.

Claims

1. Method of vapor deposition of a sulfide layer of a transition metal or of one of its alloys for the forming of MS.sub.2/MSx heterostructures or of M(M)S.sub.2 alloys with M a group-6 transition metal and M a group-5 transition metal, the method comprising a step of atomic layer deposition according to the following cycle: exposing a substrate to a precursor of a transition metal, whereby an intermediate layer is formed on the substrate, purging the reactor, exposing the intermediate layer to a precursor of sulfur, purging the reactor, the substrate being at a temperature in the range from 20 C. to 250 C. during the cycle, the cycle may be repeated several times, where the precursor of the transition metal and/or the precursor of sulfur may be identical or different during the repetitions of the cycle, the precursor of the transition metal being selected from among molybdenum oxyhalides, tungsten oxyhalides, vanadium halides, niobium halides, and tantalum halides.

2. Method according to claim 1, wherein the precursor of the transition metal is selected from among MoO.sub.2Cl.sub.2, MoOCl.sub.4, WOCl.sub.4, VCl.sub.4, NbCl.sub.5, and TaCl.sub.5.

3. Method according to claim 1, wherein the precursor of sulfur is selected from among hydrogen sulfide, hydrogen polysulfides, and thiols, preferably dithiols.

4. Method according to claim 1, wherein the precursor of sulfur is selected from among 1,2-ethanedithiol, 1,2-propanedithiol, and 1,3-propanedithiol.

5. Method according to claim 1, wherein, after the atomic layer deposition step, the method comprises a sulfurization step during which the substrate is exposed to a sulfur-containing molecule having at least one sulfur-hydrogen or sulfur-carbon bond, at a temperature in the range from 250 C. to 1,150 C., preferably from 300 C. to 400 C.

6. Method according to claim 1, wherein the sulfur molecule is a dithiol, preferably 1,2-ethanedithiol.

7. Method according to claim 1, wherein, after the atomic layer deposition step, or after the sulfurization step, an anneal step is implemented.

8. Method according to claim 1, wherein the anneal step is carried out in an inert atmosphere at a temperature in the range from 400 C. to 1,150 C., preferably from 650 C. to 950 C.

9. Method according to claim 1, wherein the substrate is at a temperature in the range from 50 C. to 150 C., preferably from 80 C. to 120 C., during the cycle.

10. Method according to claim 1, wherein the precursor of the transition metal is selected from among MoO.sub.2Cl.sub.2, MoOCl.sub.4, WOCl.sub.4, and VCl.sub.4 and wherein the precursor of sulfur is 1,2-ethanedithiol.

11. Device comprising a substrate covered by a stack of crystalline thin films forming an MS.sub.2/MSx heterostructure or by a crystalline thin film of an M(M)S.sub.2 alloy with M a group-6 transition metal and M a group-5 transition metal, the crystalline thin film(s) being of molybdenum, tungsten, vanadium, niobium, tantalum sulfide or of one of their alloys such as Mo(V)S.sub.2 or W(V)S.sub.2, the thin film(s) having a thickness smaller than 100 nm, preferably smaller than 20 nm, even more preferably smaller than 10 nm, the crystals of the thin film(s) being oriented, their (001) crystallographic plane being parallel to the plane of the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The foregoing features and advantages, as well as others, will be described in detail in the rest of the disclosure of specific embodiments given as an illustration and not limitation with reference to the accompanying drawings, in which:

[0036] FIG. 1 is a functional diagram of an ALD cycle according to a specific embodiment of the invention,

[0037] FIG. 2 is a STEM image (HAADF) of a MoS.sub.2 (1 nm)/VS.sub.x (5 nm) heterostructure obtained according to a specific embodiment of the method.

DESCRIPTION OF EMBODIMENTS

[0038] Unless otherwise specified, the expression approximately signifies to within 10%, preferably to within 5%, and the expression in the range from . . . to . . . signifies that the limits are included.

[0039] There will now be described in further detail the method of atomic layer deposition (ALD) of a thin film of a sulfide of a transition metal or of a sulfide of an alloy of transition metals. The method comprises the following steps: [0040] a) performing the ALD deposition cycle according to the following sub-steps (FIG. 1): [0041] exposing a substrate to a precursor of the transition metal, whereby an intermediate layer is formed on the substrate (sub-step i)), [0042] purging the reactor (sub-step ii)), [0043] exposing the substrate and the intermediate layer to a precursor of sulfur (sub-step iii)), whereby a sulfide layer of a transition metal is formed, [0044] purging the reactor (sub-step iv), [0045] b) preferably, carrying out a sulfurization step, for example at a temperature higher than 250 C., [0046] c) preferably, performing an anneal, advantageously in an inert atmosphere, and preferably at a temperature higher than 400 C.

[0047] Sulfurization step b) and anneal step c) may be one and the same step.

[0048] The deposition cycle may be repeated N times, with N an integer (FIG. 1). In other words, the sequence formed by sub-steps i), ii), iii), and iv) may be repeated N times.

[0049] The entire ALD sequence is carried out at low temperature, that is, at a deposition temperature lower than 250 C., and preferably at a deposition temperature in the range from 20 to 250 C., even more preferably from 50 to 250 C., even more preferably from 50 to 200 C., and even more preferably between 8 and 150 C. The deposition temperature corresponds to the substrate temperature.

[0050] Such temperatures ensure the growth of an amorphous layer in ALD regime. The deposited layer is thus perfectly uniform and of low roughness. The thickness and the morphology of the layer are advantageously identical at any point of the substrate. The low roughness guarantees that each crystal which will be formed in the layer at the end of the method will have the same thickness, and that there will be no breakage or discontinuity in the deposit.

[0051] Low growth rates during a deposition by ALD allow a better control of the composition of the layer in the case of the forming of alloys with group-5 transition metals (V, Nb, Ta).

[0052] Thus, the ALD deposition method not only allows TMD to grow in cavities with a high form factor, but also enables to control the forming of an alloy or of a heterostructure. Indeed, with such a method, it is possible, in a first step, to deposit the different materials in successive layers or as an alloy, and then to carry out the simultaneous crystallization of the different materials in a second step.

[0053] In CVD, it is more difficult to obtain alloys of controlled composition, since the precursors are introduced in a mixture and only the most thermodynamically stable composition under the temperature and partial pressure conditions used for deposition is obtained.

[0054] At step a), and more particularly during sub-step i), the precursor of the transition metal is selected from the family of oxyhalides for group-6 transition metals typically having formula MO.sub.2X.sub.2 or MOX.sub.4 (XF, Cl, Br, I) or from the family of halides for group-5 transition metals typically having formula MX.sub.n (with n in the range from 3 to 5 and XF, Cl, Br, I).

[0055] Preferably, it is selected from among molybdenum oxychlorides, tungsten oxychlorides, vanadium chlorides, niobium chlorides, and tantalum chlorides.

[0056] Such precursors are less expensive than other precursors of prior art (for example, amidides or metal-organics). Further, their greater thermal stability enables to implement the process in batch-type reactors, better adapted to large-scale production.

[0057] The saturation vapor pressure (Vp) of the precursor of metal M will advantageously be higher than 0.1 Torr, and even more preferably higher than 1 Torr, at the temperature used for the deposition process to ensure a sufficient mass transport to the reactor.

[0058] Preferably, the precursor is selected from among MoO.sub.2Cl.sub.2, MoOCl.sub.4, WOCl.sub.4, VCl.sub.4, NbCl.sub.5, and TaCl.sub.5. These precursors meet the above-mentioned volatility criterion (Vp>0.1 Torr at the deposition temperature).

[0059] As a result of sub-step i), an intermediate layer comprising the transition metal or an intermediate molecule comprising the transition metal is formed on the substrate. This intermediate layer will react with the sulfur-containing molecule during sub-step iii).

[0060] During sub-step iii), the sulfur precursor is selected from among hydrogen sulfide, a hydrogen polysulfide, an organosulfur compound containing, preferably, at least 2 sulfur-hydrogen bonds, or any other system allowing the in-situ forming of the mentioned precursors (plasma generator or so-called thermal pre-cracking unit, for example).

[0061] Preferably, the sulfur precursor is selected from among hydrogen sulfide, a hydrogen polysulfide, and thiols, preferably dithiols.

[0062] Even more preferably, the dithiol is selected from among 1,2-ethanedithiol, 1,2-propanedithiol, and 1,3-propanedithiol.

[0063] The precursors of the transition metal or of sulfur may be in solid, liquid, or gaseous form. Precursors in solid or liquid form are stored in a stainless steel saturator.

[0064] Preferably, the precursors are introduced into the reactor in vapor form (gas). The temperature to which the precursors are heated in the saturator depends on their volatility. The temperature will be selected so as to reach a sufficient vapor pressure to feed the reactor (typically from approximately 0.1 to 5 torr, that is, between approximately 13.3 and 666.6 Pa).

[0065] It is possible to perform the ALD cycle with the same precursors or with different precursors during the different repetitions of the cycle.

[0066] For example, to form Mo(V)S.sub.2 alloys or MoS.sub.2/VS.sub.2 heterostructures during the ALD cycle, the MoO.sub.2Cl.sub.2/EDT and/or VCl.sub.4/EDT pairs of precursors may be used.

[0067] The deposition temperature of VS.sub.2 is preferably identical to the deposition temperature of MoS.sub.2 (lower than 150 C.) and the same sulfur precursor (EDT) is used, which enables to carry out Mo(V)S.sub.2 deposition sequences where the proportion of vanadium can be perfectly controlled.

[0068] Preferably, the sulfur precursor is identical all throughout the process. Preferably, it is 1,2-ethanedithiol (EDT).

[0069] The ALD cycle is implemented in a reactor allowing a sequential feeding of the precursors. The precursors are not introduced concomitantly. A purge (sub-steps ii) and iv)) is carried out between each introduction of precursors to drain off precursors which have not reacted during the previous sub-step as well as volatile reaction by-products. The purge is performed with a neutral gas, such as argon or nitrogen.

[0070] The working pressure is preferably in the range from 1 mTorr and 50 Torr (that is, between approximately 0.1 Pa and 6,666.1 Pa), and even more preferably between 0.1 and 10 Torr. The working pressure may vary according to the volume and to the sizing of the reactor.

[0071] The substrate is, for example, a silicon substrate (in particular a silicon wafer) covered with a thin layer of silica or of any other oxide, nitride, or metal material having a low surface roughness and which does not react with the deposited TMD layer or the reactants used during the deposition, sulfurization, or annealing steps. Alternatively, the growth may take place on another TMD (sulfide, selenide, or tellurium). The exposed surface of the substrate may comprise different areas formed of the different previously-mentioned materials, with a view to the integration of the TMD layer into a microelectronic device.

[0072] As already mentioned, the deposit obtained at the end of step a) is amorphous. It is a coordination polymer containing metal-sulfur bonds as well as 1,2-ethanedithiolato ligands. Such a polymer is converted into a highly uniform sulfide layer during the anneal step (step c)).

[0073] At the end of step a), depending on the temperature and on the reactants used during the ALD cycle, the obtained thin film may still contain unsubstituted ligands or carbon present in the sulfur precursor. The optional sulfurization step (step b), which can be carried out before or during step c)), enables to guarantee the removal of residual ligands, in other words, that the thin film is exclusively formed of metal-sulfur bonds, and the obtaining of a sulfide with the correct stoichiometry. Another advantage of the sulfurization step is to pre-crystallize the TMD thin film and to give it a better stability in air which enables to limit the forming of metal-oxygen bonds during the transfer to the thermal annealing equipment, and/or, if necessary, during the steps of cleaning of the back side of the substrate.

[0074] The temperature of step b) is preferably at a temperature higher than 250 C., preferably at a temperature in the range from 250 to 1,150 C., even more preferably from 300 to 400 C.

[0075] The sulfurization step is carried out in the presence of a sulfur-containing compound. The precursors used for this step may be sulfur in its native form or any volatile molecule containing SH or SC bonds, used as a dilute vapor in an inert gas, or in a mixture with hydrogen.

[0076] Preferably, it may be the same sulfur-containing molecule as that used in the deposition step, which then enables to carry out the sulfurization step directly in the equipment used for the deposition, without venting. For example, the sulfurization of the thin film obtained with the MoO.sub.2Cl.sub.2/EDT pair of precursors to form MoS.sub.2 may be carried out optimally at 360 C. under EDT vapor for a 30 min time period.

[0077] Sulfurization step b) may be directly applied to the final alloy or heterostructure. There is no need to repeat a separate sulfurization step between each layer of a different metal. Indeed, the diffusion of sulfur in an amorphous TMD layer is more than sufficient to ensure the sulfurization of the thin layers of a thickness of some ten nanometers within a few minutes.

[0078] The implementation of step c) depends on the temperatures used during steps a) and/or b).

[0079] Thermal anneal step c) enables to crystallize the TMDs thin film and/or to improve its crystalline quality. It ensures the forming of TMDs crystals oriented in the plane of the substrate and of optimum size.

[0080] This step is preferably carried out in an inert atmosphere (N.sub.2, He, or Ar, in particular).

[0081] This anneal is carried out at a temperature higher than that used during sulfurization step b). The anneal is typically carried out at a temperature in the range from 400 to 1,150 C., ideally from 650 C. to 950 C.

[0082] For example, the temperature of step c) is advantageously 900 C. for MoS.sub.2 or WS.sub.2, such as for example in the case where the targeted application requires an optimum crystallinity of the TMD.

[0083] The heat source may be a resistor or any other emissive source (halogen lamp or laser) that can be absorbed by the TMD thin film or any other constituent of the growth substrate.

[0084] With such a method, it is possible to manufacture thin films of MS.sub.2 or MSx with M a group-6 transition metal and M a group-5 transition metal, M(M)S.sub.2-type alloys, or also heterostructures consisting of a stack of different TMD materials (MS.sub.2/MS.sub.x for example).

[0085] More particularly, the obtained device comprises a substrate covered by a thin film or a stack of crystalline thin films of molybdenum, tungsten, vanadium sulfide (VS.sub.2 or VS.sub.x with x greater than 1 and smaller than 3), niobium, tantalum sulfide, or one of their alloys such as Mo(V)S.sub.2 or W(Nb)S.sub.2.

[0086] With such a method, it is possible to obtain layers of different thicknesses according to the targeted application.

[0087] The thin film may have a thickness ranging up to 50 nm, or even up to 100 nm, for example, to form metallic contacts made of TMDs (group 5).

[0088] The thin layer may have a smaller thickness. For example, it may be a thickness smaller than 20 nm, preferably smaller than 10 nm.

[0089] The minimum thickness of the thin film may correspond to the thickness of an atomic monolayer according to the 001 plane, that is, for example 0.65 nm for a MoS.sub.2 monolayer.

[0090] The obtained thin films may have very low roughness. The RMS roughness (determined by AFM) may typically be lower than 0.3 nm.

[0091] The method is particularly advantageous because it enables to obtain ultra-thin (typically having a thickness smaller than 5 nm), smooth (of low roughness; typically having a roughness lower than 0.3 nm), and continuous layers.

[0092] The (001) planes of the TMD crystals are oriented parallel to the plane of the substrate.

[0093] The method is easy to industrialize due to its low cost and to the thermal stability of the precursors used.

[0094] The method is particularly advantageous to manufacture (micro) electronic devices such as field-effect transistors, memristors, RF switches, and devices for spintronics or quantum computing.

[0095] Vanadium, niobium, and tantalum enable to induce a robust p-type doping in MoS.sub.2 and WS.sub.2 materials. Further, VS.sub.2 exhibits a near-ideal lattice match with MoS.sub.2 and WS.sub.2.

[0096] It is thus possible to form VS.sub.2/MoS.sub.2 or VS.sub.2/WS.sub.2 heterostructures with very low stress, or Mo(V)S.sub.2 or W(V)S.sub.2 semiconductor alloys capable of exhibiting more advantageous conduction properties than pure MoS.sub.2 and WS.sub.2.

[0097] Lamellar vanadium sulfides (VS.sub.2 and V.sub.5S.sub.8) are good conductors (resistivities lower than one mOhm.Math.cm). In particular, VS.sub.2 has a lower contact resistance on MoS.sub.2. The implementation of a sulfide-based contact is also particularly advantageous to avoid damaging a TMD semiconductor based on MoS.sub.2 or WS.sub.2.

[0098] Various embodiments and variants have been described. The person skilled in the art will understand that certain features of these various embodiments and variants could be combined, and other variants will become apparent to the person skilled in the art.

[0099] Finally, the practical implementation of the modes of realization and variants described is within the reach of the person skilled in the art, based on the functional indications given above.

ILLUSTRATIVE AND NON-LIMITING EXAMPLES OF DIFFERENT EMBODIMENTS

Example 1: Deposition of a MoS.SUB.2 .Layer

[0100] The ALD cycle is performed by successively alternating pulses of the MoO.sub.2Cl.sub.2 and EDT (1,2-ethanedithiol) precursors at 100 C.

[0101] The temperature of the MoO.sub.2Cl.sub.2 source is 68 C. The temperature of the EDT source is 40 C.

[0102] After the forming of the thin film, a sulfurization step is carried out in the presence of EDT for 30 min at 360 C. A rapid thermal anneal (RTP) under N.sub.2 for 30 seconds at 900 C. results in the crystallization of the MoS.sub.2 thin film.

Example 2: Deposition of a Vanadium Sulfide Layer

[0103] The ALD cycle is performed by successively alternating pulses of the VCl.sub.4 and EDT (1,2-ethanedithiol) precursors at 100 C.

[0104] The temperature of the VCl.sub.4 source is 30 C. The temperature of the EDT source is 40 C.

[0105] After the forming of the thin layer, a sulfurization step is carried out in the presence of EDT for 30 min at 360 C., with no venting between the deposition and sulfurization steps.

[0106] The resistivity of the vanadium sulfide layer obtained after sulfurization is approximately 1,000 Ohm.Math.cm for a 10-nm thickness.

[0107] A thermal anneal after the sulfurization step enables to crystallize the material and to give it a better resistance to oxidation. The material keeps its metallic properties at annealing temperatures ranging up to 950 C., with a non-linear resistivity variation.

Example 3: Deposition of a Mo(V)S.SUB.2 .Layer (Vanadium-Doped Molybdenum Sulfide)

[0108] The layer is obtained by ALD by using the MoO.sub.2Cl.sub.2, VCl.sub.4, and EDT (1,2-ethanedithiol) precursors at a 100 C. temperature. The precursors are introduced according to sequence [(MoO.sub.2Cl.sub.2/EDT).sub.X/(VCl.sub.4/EDT)].sub.Y with x and y positive integers.

[0109] The temperature of the MoO.sub.2Cl.sub.2 source is 68 C. The temperature of the VCl.sub.4 source is 30 C. The temperature of the EDT source is 40 C.

[0110] Once the forming of the thin layer, a sulfurization step is carried out in the presence of EDT for 30 min at 360 C.

[0111] A rapid thermal anneal (RTP) under N.sub.2 for 30 seconds at 900 C. results in the crystallization of the Mo(V)S.sub.2 thin film. The resistivity of the MoS.sub.2 layer is all the smaller as the amount of incorporated vanadium is significant.

Example 4: Forming of a MoS.SUB.2./VS.SUB.X .Heterostructure

[0112] The layers of the heterostructure were deposited by ALD at 100 C. by abutting the sequences described in examples 1 and 2 and adjusting the number of ALD cycles to obtain 2 atomic monolayers of MoS.sub.2 coated with 5 nm of vanadium sulfide. The heterostructure was then sulfurized at 350 C. and crystallized by rapid thermal anneal at 850 C.

[0113] The obtained heterostructure was characterized by transmission electron microscopy (STEM-HAADF), thus highlighting the forming of a MoS.sub.2 (1 nm)/VS.sub.x (5 nm) stack (FIG. 2).