Abstract
In one aspect, a method of depositing a transition metal dichalcogenide is provided. The method includes depositing a layer of the transition metal dichalcogenide on a substrate by a metalorganic chemical vapor deposition process including exposing the substrate to a mixture of reactant gases including a transition metal precursor and a chalcogen precursor. The mixture further includes a gas-phase halogen-based reactant to volatilize transition metal adatoms deposited on the substrate.
Claims
1. A method of depositing a transition metal dichalcogenide, the method comprising: depositing a layer of the transition metal dichalcogenide on a substrate by a metalorganic chemical vapor deposition process comprising exposing the substrate to a mixture of reactant gases comprising a transition metal precursor and a chalcogen precursor, wherein the mixture further comprises a gas-phase halogen-based reactant to volatilize transition metal adatoms deposited on the substrate.
2. The method according to claim 1, wherein in the metalorganic chemical vapor deposition process, a deposition temperature and a gas-phase halogen-based reactant concentration of the mixture are such that a desorption flux from the substrate exceeds a net condensation flux to the substrate.
3. The method according to claim 1, wherein a deposition temperature of the metalorganic chemical vapor deposition process is 750? C. or lower.
4. The method according to claim 1, wherein the substrate is arranged in a reactor and wherein the gas-phase halogen-based reactant is introduced into the reactor simultaneous to the transition metal precursor and chalcogen precursor.
5. The method according to claim 4, wherein the gas-phase halogen-based reactant is introduced into the reactor in an amount exceeding an amount of the transition metal precursor introduced into the reactor.
6. The method according to claim 1, wherein the gas-phase halogen-based reactant is HCl or Cl.sub.2.
7. The method according to claim 1, wherein the transition metal precursor comprises W, Mo, Zr, or Hf.
8. The method according to claim 1, wherein the transition metal precursor is a carbonyl (M-(CO).sub.x), an alkyl (M-R.sub.x), or an alkoxide (M-(OR).sub.x).
9. The method according to claim 1, wherein the chalcogen precursor comprises S, Se, or Te.
10. The method according to claim 1, wherein the chalcogen precursor is H.sub.2S, H.sub.2Se, or H.sub.2Te.
11. The method according to claim 1, wherein the layer of the transition metal dichalcogenide is deposited on an amorphous or crystalline surface of the substrate.
12. The method according to claim 11, wherein the surface of the substrate is a sapphire surface.
13. The method according to claim 11, wherein the surface of the substrate is a surface of a scaled dielectric layer of the substrate.
14. The method according to claim 13, wherein the dielectric layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, HfO.sub.2, ZrO.sub.2, Si.sub.3N.sub.4, or SiCO.
15. The method according to claim 1, wherein the transition metal precursor comprises W, the chalogen precursor comprises S, and the gas-phase halogen-based reactant is HCl.
16. The method according to claim 15, wherein the transition metal precursor is W(CO).sub.6 and the chalogen precursor is H.sub.2S.
17. The method according to claim 1, wherein a nucleation density is less than 1 ?m.sup.?2.
18. The method according to claim 17, wherein micrometer size transition metal dichalcogenide crystals without grain boundaries are grown.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The above, as well as additional objects, features and advantages, may be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.
[0028] FIGS. 1A, 1B, 1C, and 1D illustrate, as a comparative example, challenges with MOCVD of MX.sub.2.
[0029] FIGS. 2A-2B and 3A-3B schematically illustrate deposition in a case of high and low supersaturation, respectively.
[0030] FIG. 4 schematically depicts a deposition method according to an embodiment.
[0031] FIG. 5 shows the result of HCl-assisted MOCVD deposition of WS.sub.2 according to an embodiment of the disclosed technology.
[0032] FIGS. 6A, 6B, 6C, and 6D depict an influence of relative HCl concentration on the MOCVD deposition of WS.sub.2.
[0033] FIG. 7 shows metal precursor desorption rate as a function of deposition temperature for MOCVD of WS.sub.2 with and without introduction of an HCl reagent.
DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS
[0034] FIGS. 1A-1D illustrate as a comparative example, challenges with MOCVD of MX.sub.2. FIGS. 1A-1D show images obtained using an atomic force microscope (AFM) of the result of a 10 minutes MOCVD process of WS.sub.2 using the W(CO).sub.6 and H.sub.2S precursors at four different deposition temperatures 700? C., 800? C., 900? C. and 1000? C. respectively. The starting surface is in each example a 2000 nm SiO.sub.2 (for example, grown by wet thermal oxidation of a Si wafer). In practice, only the deposition temperature of 700? C. may be considered Fab-compatible given the reactivity of precursors towards starting surface. However, this results in a very high nucleation density, which may be attributed to a low tungsten adatom desorption and diffusion length. Since surface diffusion is mainly temperature driven, higher deposition temperatures provide a reduced nucleation density and increased crystal size as may be seen in FIGS. 1A-1D. However, deposition temperatures of 800? C. further exhibit an increasing co-deposition of tungsten (seen as white dots).
[0035] FIGS. 2A and 2B schematically illustrate deposition under a condition of high supersaturation and low supersaturation, respectively, for a transition metal precursor: A denotes adsorption to the substrate surface, D denotes desorption from the substrate surface, SD denotes surface diffusion, and GD denotes gas diffusion. It is envisaged that the process of adsorption, as discussed herein, may be understood to refer to adsorption by physisorption, wherein the adatoms are bonded to the starting surface of the substrate by weak van der Waals forces. High supersaturation may refer to a desorption flux that is much lower than a net condensation flux to the substrate surface, as shown in FIG. 3A. High supersaturation corresponds to the comparative examples shown in FIGS. 1A-1D, wherein the adsorption is substantially irreversible and the transition metal adatoms are incorporated into the MX.sub.2 crystal predominantly through surface diffusion. On the other hand, low supersaturation may refer to a desorption flux that is appreciably greater than a net condensation flux to the substrate surface, as shown in FIG. 3B. As may be seen in FIG. 3B, under low supersaturation, the adsorption and desorption fluxes may be substantially equal.
[0036] As set out herein, embodiments of the disclosed technology provide a method which enables deposition of MX.sub.2 using MOCVD under a low supersaturation regime. More specifically, according to an aspect of the disclosed technology, there is provided a method of depositing a transition metal dichalcogenide (for example, MX.sub.2 material). The method includes depositing a layer of MX.sub.2 on a substrate by a MOCVD process including exposing the substrate to a mixture of reactant gases including a transition metal precursor (for example, a transition metal in a metalorganic precursor) and a chalcogen precursor. The mixture further includes a gas-phase halogen-based reactant to volatilize transition metal adatoms deposited on the substrate.
[0037] FIG. 4 schematically depicts an embodiment of a method according to the present disclosure, wherein a substrate 104 is arranged in a reactor 102 of an MOCVD tool 100. The substrate 104 provides the starting surface for the MX.sub.2 deposition and may be formed by a surface of an amorphous dielectric layer, for example, a scaled dielectric layer. The dielectric layer may for example be a layer of SiO.sub.2, Al.sub.2O.sub.3, HfO.sub.2, ZrO.sub.2, Si.sub.3N.sub.4, or SiCO, for example, with a thickness in a range from 5-10 nm. However, the starting surface may also be formed by a sapphire substrate, or more generally, another suitable monocrystalline, polycrystalline or amorphous substrate.
[0038] As depicted in FIG. 4, the transition metal precursor (for example, W(CO).sub.6), the chalcogen precursor (for example, H.sub.2S) and the halogen-based reactant (for example, HCl) are simultaneously introduced in gas-phase into the reactor 102 to form a mixture of reactant gases. The precursors and the reactant may be introduced in an inert carrier gas, such as argon (Ar), helium (He), or N.sub.2. The substrate 104 may thus be simultaneously exposed to the constituent gases of the mixture. The substrate 104 may be heated (for example, by a heating element of a substrate holder, and/or by heating the atmosphere in the reactor) to provide a reaction condition enabling a gradual and sustained deposition of MX.sub.2 on the starting surface of the substrate 102 for the duration of the MOCVD process (for example, on the order of a few minutes, depending on a desired size of the MX.sub.2 layers). An example of an overall deposition reaction is M(CO).sub.6+2 H.sub.2S+HCl->MS.sub.2+6 CO+2 H.sub.2+HCl (for example, for excess of HCl), where M denotes the transition metal species.
[0039] In various implementations, the simultaneous presence of the precursors and the halogen-based reactant enables halogenation (for example, chlorination) of both transition metal adatoms (for example, W adsorbed to the substrate 102) and the transition metal precursor (for example, W(CO).sub.6). Thereby, the transition metal precursor may be volatilized to enable reversible precursor adsorption, and diffusion by lateral gas-phase transport may be enhanced (compare, for example, FIGS. 2B and 3B).
[0040] FIG. 7 shows as an illustrative example respective simulated metal precursor desorption rates as a function of deposition temperature for MOCVD of WS.sub.2 from W(CO).sub.6, and H.sub.2S for an HCl reagent concentration of 0%, 0.1%, 1% and 10%. As may be seen, HCl-assisted MOCVD considerably increases the desorption rate already at relatively minute amounts, and enables low supersaturation to be achieved during the deposition at Fab-compatible deposition temperatures, for example, below 750? C.
[0041] In addition to controlling the deposition temperature, the reaction conditions (and hence the nucleation density and net condensation) may be controlled by the relative concentrations of the transition metal precursor, the chalcogenide precursor and the halogen reactant (for example, by controlling the flow via respective inlet and outlet valves of the MOCVD tool). To facilitate achieving a condition of low supersaturation, the halogen-based reactant HCl may advantageously be introduced into the reactor 102 in an amount exceeding an amount of the transition metal precursor W(CO).sub.6.
[0042] While FIG. 4 shows a mixture of W(CO).sub.6, H.sub.2S and HCl, this is merely an example and other combinations of precursors and halogens are also possible, as discussed above.
[0043] FIG. 5 is an SEM image of a micrometer-sized WS.sub.2 crystal deposited on a SiO.sub.2 starting surface using a method in accordance with the disclosed technology. The WS.sub.2 was deposited in an MOCVD process using W(CO).sub.6 and H.sub.2S precursors and an HCl reactant. A concentration of W(CO).sub.6 was 0.3 ppm, while the flow of H.sub.2S was 20 sccm, and the flow of HCl was 1500 sccm for a total gas flow of 25000 sccm. A carrier gas of argon was used. The process was performed at a reactor pressure of 32 Torr and a deposition temperature of 750? C. This example demonstrates how the method of the disclosed technology can combine the ease of use of MOCVD precursors (for example, in terms of vapor pressure and precursor delivery) and volatility of halogens in a Fab-compatible deposition process.
[0044] FIGS. 6A-6D show the influence of the relative concentration of HCl in the mixture of reactant gases for a fixed deposition temperature of 750? C. As the HCl concentration is increased, the nucleation density as well as transition metal co-deposition is suppressed. As seen in FIG. 6D, a relative HCl concentration of 6.0% allows the nucleation density to be reduced to less than 1 ?m.sup.?2, wherein growth of micrometer size MX.sub.2 crystals without grain boundaries is facilitated.
[0045] In the above, the disclosed technology has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the disclosed technology.
