Semiconductor structure with template for transition metal dichalcogenides channel material growth

Disclosed is a semiconductor structure comprising a single crystal substrate, a channel layer formed above the substrate from a transition metal dichalcogenides (TMDC) material, and a single crystal epitaxial buffer layer formed between the substrate and the channel layer, wherein the buffer layer is formed from material having a lattice constant mismatch of less than 5% with the lattice constant of the channel layer material. Also, disclosed is a method of forming a semiconductor structure comprising selecting a substrate formed from a single crystal material, preparing the substrate for template growth, growing a template on the substrate wherein the template is formed from single crystal material, and growing channel material on the template wherein the channel material is formed from a TMDC material and wherein the buffer layer material has a lattice constant mismatch of less than 5% with the lattice constant of the channel layer material.

BACKGROUND

The technology described in this patent document relates to transition metal dichalcogenides, and more specifically to forming semiconductor structures with transition metal dichalcogenides material as channel material.

Scaling of semiconductor devices, such as a MOSFET, has enabled continued improvement in speed, performance, density, and cost per unit function of integrated circuits over the past few decades. Two-dimensional transition metal dichalcogenides (TMDC) materials such as MoS2exhibit quantum confinement effect and are a promising channel material for FETs in low power consumption applications.

DETAILED DESCRIPTION

Two-dimensional transition metal dichalcogenides (TMDC) materials such as MoS2, exhibit desirable characteristics such as high channel mobility (˜200 cm2/Vs in some examples), a high current ON/OFF ratio (˜108in some examples), and a good sub-threshold swing (˜70 mV per decade in some examples). It may be desirable to use TMDC materials as channel material in FETs. Developing a large-area growth method for TMDC materials can make it easier to fabricate FETs with TMDC material as channel material. Semiconductor structures and methods for fabricating semiconductor structures are disclosed which provide TMDC materials as channel material.

FIG. 1Adepicts an example semiconductor structure10with TMDC material as channel material. The semiconductor structure10comprises a single crystal substrate12. Examples of single crystal substrate materials that may be used include Si(111), 4H—SiC(0001), sapphire(0001) and AlN(0001). In this example, the single crystal substrate12comprises sapphire(0001).

The semiconductor structure10also includes a layer of channel layer material16from which the channel of a transistor may be formed. The channel layer16is formed above the substrate12from a transition metal dichalcogenides (TMDC) material. TMDCs are a class of materials with the chemical formula MX2, where M represents a transition metal element from group IV (e.g., Ti, Zr, Hf, etc.), group V (e.g., V, Nb or Ta) or group VI (e.g., Mo, W, etc.). The ternary compound of transition metal elements to form dichalcogenides is also included such as MoWS2, etc. The X represents a chalcogen (e.g., S, Se or Te). In this example, the channel material comprises MoS2.

The semiconductor structure10further includes a buffer layer14formed between the substrate12and the channel layer16. The buffer layer14is a single crystal epitaxial buffer layer formed between the substrate and the channel layer16. The buffer layer14is formed from material having a lattice constant mismatch of less than 5% with the lattice constant of the channel layer material. The buffer layer may be formed from materials such as AlN(0001), GaN(0001), InGaN(001), InAlN(0001), BN(0001), TiN(111), and SiC(0001), which may reduce misfit dislocation density for the TMDC channel material. In this example, the buffer layer comprises AlN(0001).

FIG. 1Bdepicts a cross-sectional view of the example semiconductor structure10. In this example, the substrate12comprises sapphire(0001), the buffer layer14comprises AlN(0001), and the channel material16comprises MoS2.

FIG. 2is a process flow diagram illustrating an example process for forming a semiconductor structure with TMDC material as channel material. A substrate and template for channel material growth are selected (operation102). The substrate is formed from a single crystal material, such as Si(111), 4H—SiC(0001), sapphire(0001) or other single crystal substrate (option103). In this example, the substrate is also cleaned.

After substrate selection, the substrate is prepared for template growth (operation104). Preparation may involve pre-treating the substrate with hydrogen gas in a vacuum environment at a high temperature (option105).

After substrate preparation, a template is grown on the substrate as a buffer layer (operation106). The buffer layer is formed from a single crystal material. In the example illustrated, a single crystal AlN epitaxial buffer layer is deposited on the substrate as a template for MoS2growth (option107). The growth method of the buffer layer can be MOCVD, sputtering or pulsed laser deposition, among others.

After template growth, channel material is grown on the template (operation108). In this example, the channel material comprises MoS2(option109). A transistor or other semiconductor device can be fabricated using the semiconductor structure formed by the example process.

FIG. 3is a process flow diagram illustrating another example process for forming a semiconductor structure with TMDC material as channel material. A substrate and template for channel material growth are selected (operation112). In this example, the substrate is formed from a single crystal material and undergoes two cleanings in hot acetone and a cleaning using hot isopropyl alcohol (option113).

After substrate selection, the substrate is prepared for template growth (operation114). Preparation may involve pre-treating the substrate with a hydrogen gas (H2) purge in a vacuum environment at a temperature of around 800-1000° C. (option115).

After substrate preparation, a template is grown on the substrate as a buffer layer (operation116). In the example illustrated, a c-plane AlN single crystal epitaxial layer is grown on the substrate with a thickness of about 2 nm to 200 nm (option117).

After template growth, channel material is grown on the template (operation118). A transistor or other semiconductor device can be fabricated using the semiconductor structure formed by the example process.

FIG. 4is a process flow diagram illustrating another example process for forming a semiconductor structure with TMDC material as channel material. A substrate and template for channel material growth are selected (operation122). The substrate, in this example, comprises 2″ c-plane sapphire (option123).

After substrate selection, the substrate is prepared for template growth (operation124). Preparation may involve loading the substrate into a metal organic chemical vapor deposition (MOCVD) vacuum chamber for AlN growth (option125) and pre-treating the substrate with a hydrogen gas (H2) flow in the vacuum chamber at a temperature of around 950° C. (option127).

After substrate preparation, a template is grown on the substrate as a buffer layer (operation126). In the example illustrated, an AlN single crystal epitaxial layer is grown in the MOCVD vacuum chamber at a temperature of around 900-1000° C. by Al precursor Trimethylaluminum (referred to as TMA or (CH3)3Al) and nitrogen precursor Ammonia (NH3) (option129).

After template growth, the surface of AlN can be treated by a sulfurization process to form a sulfur termination on the template surface. The sulfurization process may comprise pretreatment using H2S gas or Sulfur vapor ambient at a temperature of around 500-1000° C.

After template growth, channel material is grown on the template (operation128). In this example, channel material growth involves pulse laser deposition in a chamber wherein the ablation source is a 248 nm wavelength excimer laser, the chamber pressure is less than 1 E-5 torr, and the target is hot pressed and sintered MoS2of a 1 inch diameter (option131). Ablation is performed at a temperature of approximately 500-900° C. and a laser energy density of approximately 1-5 J/cm2(option133). A transistor or other semiconductor device can be fabricated using the semiconductor structure formed by the example process.

In any of the foregoing example processes, the channel material may be formed from a transition metal dichalcogenides (TMDC) material, wherein the buffer layer material has a lattice constant mismatch of less than 5% with the lattice constant of the channel layer material. In any of the foregoing example processes, the substrate may comprise a Si(111), 4H—SiC(0001), or sapphire(0001) single crystal substrate.

FIG. 5depicts a top down view of an example semiconductor structure200formed using one or more of the example processes. The example semiconductor structure200comprises a single crystal substrate202. Examples of single crystal substrate materials include Si(111), 4H—SiC(0001), sapphire(0001) and AlN(0001).

The semiconductor structure200also includes a layer of channel layer material204from which the channel of a transistor may be formed. The channel layer204is formed above the substrate202from a transition metal dichalcogenides (TMDC) material. The chemical formula of the TMDC channel material comprises MX2, wherein the M represents a transition metal element from group IV, group V, or group VI, and the X represents a chalcogen such as S, Se or Te.

The semiconductor structure200further includes a buffer layer206formed between the substrate202and the channel layer204. The buffer layer206is a single crystal epitaxial buffer layer formed between the substrate and the channel layer204. The buffer layer206is formed from material having a lattice constant mismatch of less than 5% with the lattice constant of the channel layer material. The buffer layer was grown on the substrate by metal organic chemical vapor deposition (MOCVD), sputtering or pulsed laser deposition. The buffer layer may comprise AlN(0001), GaN(0001), InGaN(001), InAlN(0001), BN(0001), TiN(111), or SiC(0001).

Also illustrated are transistor terminals for the semiconductor structure. Shown are a source region208, a gate region210, and a drain region212. In the example illustrated, the substrate comprises sapphire(0001), the buffer layer comprises AlN(0001), and the channel material comprises MoS2. Also, in the example illustrated, the substrate was pre-treated with hydrogen gas in a vacuum at high temperature prior to buffer layer formation.

The methods and structures disclosed herein can provide easier MoS2nucleation and epitaxy, a high crystalline quality of MoS2growth, and may be suitable for large area epitaxy.

In one embodiment, disclosed is a semiconductor structure comprising a single crystal substrate, a channel layer formed above the substrate from a transition metal dichalcogenides (TMDC) material, and a single crystal epitaxial buffer layer formed between the substrate and the channel layer. The buffer layer is formed from material having a lattice constant mismatch of less than 5% with the lattice constant of the channel layer material.

These aspects and other embodiments may include one or more of the following features. The chemical formula of the TMDC channel material may comprise MX2, wherein the M represents a transition metal element from group IV, group V, or group VI, and the X represents a chalcogen such as S, Se or Te. The channel material may comprise MoS2. The buffer layer may comprise AlN(0001), GaN(0001), InGaN(001), InAlN(0001), BN(0001), TiN(111), or SiC(0001). The substrate may comprise Si(111), 4H—SiC(0001), sapphire(0001) or AlN(0001).

In another embodiment, disclosed is a method of forming a semiconductor structure comprising selecting a substrate wherein the substrate is formed from a single crystal material, preparing the substrate for template growth, growing a template as a buffer layer on the substrate wherein the buffer layer is formed from a single crystal material, and growing channel material on the template wherein the channel material is formed from a transition metal dichalcogenides (TMDC) material and wherein the buffer layer material has a lattice constant mismatch of less than 5% with the lattice constant of the channel layer material.

These aspects and other embodiments may include one or more of the following features. The preparing the substrate for template growth may comprise pre-treating the substrate with hydrogen gas in a vacuum at high temperature. The preparing the substrate for template growth may comprise loading the substrate into a vacuum chamber and pre-treating the substrate with a H2purge at 800-1000° C. The preparing the substrate for template growth may comprise loading the substrate into a metal organic chemical vapor deposition (MOCVD) vacuum chamber and pre-treating the substrate with a hydrogen gas flow at a temperature of approximately 950° C. The substrate may comprise a Si(111), 4H—SiC(0001), or sapphire(0001) single crystal substrate. The selecting a substrate may comprise cleaning a single crystal substrate with hot acetone and also with isopropyl alcohol. The selecting a substrate may comprise selecting a 2-inch c-plane sapphire as the substrate. The growing a template as a buffer layer may comprise growing the buffer layer by MOCVD, sputtering or pulsed laser deposition. The growing a template as a buffer layer may comprise growing the buffer layer by MOCVD at a temperature of around 900-1000° C. The growing a template as a buffer layer may comprise performing a sulfurization process to form a sulfur termination on a surface of the buffer layer. The performing a sulfurization process may comprise pretreatment using H2S gas or Sulfur vapor ambient. The performing a sulfurization process may comprise pretreatment at a temperature of around 500˜1000° C.

In yet another embodiment, disclosed is a semiconductor structure comprising a single crystal substrate wherein the substrate comprises Si(111), 4H—SiC(0001), sapphire(0001) or AlN(0001), a channel layer formed above the substrate from a transition metal dichalcogenides (TMDC) material wherein the chemical formula of the TMDC channel material comprises MX2. The M represents a transition metal element from group IV, group V, or group VI, and the X represents a chalcogen. The semiconductor structure further comprises a single crystal epitaxial buffer layer formed between the substrate and the channel layer. The buffer layer is formed from material having a lattice constant mismatch of less than 5% with the lattice constant of the channel layer material. The buffer layer was grown on the substrate by metal organic chemical vapor deposition (MOCVD), sputtering or pulsed laser deposition.

These aspects and other embodiments may include one or more of the following features. The substrate may comprise sapphire(0001), the buffer layer may comprise AlN(0001), and the channel material may comprise MoS2. The substrate may have been pre-treated with hydrogen gas in a vacuum at high temperature prior to buffer layer formation. The buffer layer may comprise GaN(0001), InGaN(001), InAlN(0001), BN(0001), TiN(111), or SiC(0001)) and may have been pre-treated with sulfurization before TMDCs materials growth.