Scalable Substitutional Re‐Doping and its Impact on the Optical and Electronic Properties of Tungsten Diselenide

Abstract

Doping is the cornerstone of semiconductor technology, enabling the success of modern digital electronics. 2D transition metal dichalcogenides (TMDCs) are promising material platforms for future electronics applications where its wafer-scale synthesis and controllable doping will be a required prerequisite to drive the next technological revolution. [1-6] Successful realization of wafer-scale, electronic grade, intrinsic 2D TMDCs via common deposition methods is rapidly progressing, however, advances in scalable doping still remain in the "proof-of-concept" stage, delaying the largescale fabrication of logic circuits based on extrinsic 2D semiconductors. [7-12] Moreover, integration of 2D TMDCs with Si complementary metal-oxide-semiconductor at the back-end-of-line (BEOL) is being actively explored for diffusion barriers, liners, and thin-film-transistors to improve the overall integrated circuit performance. [13-16] However, BEOL production lines are Reliable, controlled doping of 2D transition metal dichalcogenides will enable the realization of next-generation electronic, logic-memory, and magnetic devices based on these materials. However, to date, accurate control over dopant concentration and scalability of the process remains a challenge. Here, a systematic study of scalable in situ doping of fully coalesced 2D WSe 2 films with Re atoms via metal-organic chemical vapor deposition is reported. Dopant concentrations are uniformly distributed over the substrate surface, with precisely controlled concentrations down to <0.001% Re achieved by tuning the precursor partial pressure. Moreover, the impact of doping on morphological, chemical, optical, and electronic properties of WSe 2 is elucidated with detailed experimental and theoretical examinations, confirming that the substitutional doping of Re at the W site leads to n-type behavior of WSe 2. Transport characteristics of fabricated back-gated fieldeffect-transistors are directly correlated to the dopant concentration, with degrading device performances for doping concentrations exceeding 1% of Re. The study demonstrates a viable approach to introducing true dopantlevel impurities with high precision, which can be scaled up to batch production for applications beyond digital electronics.

Controllable p‐Type Doping of 2D WSe2 via Vanadium Substitution

“…Furthermore, an increase in density of bilayer islands occurs in extrinsic films with increasing dopant concentration (Figure S1c,d, Supporting Information), suggesting that vanadium adatoms serve as secondary nucleation centers on the surface. [18] In parallel, BEOL-compatible WSe 2 is obtained on SiO 2 /Si substrates at 400 °C and 700 Torr, with all films being coalesced, multilayer, and polycrystalline with an average domain size of 100 nm and an average thickness of 2.11 nm, corresponding to 3-4 layers (Figures S2a-d and S3a,b, Supporting Information). [11] Moreover, chemical composition analysis via XPS indicates that V is incorporated into the WSe 2 lattice at both FEOL and BEOL temperatures.…”

“…Pristine and V-doped FEOL compatible WSe 2 is synthesized on c-plane sapphire substrates at 800 °C and 700 Torr using W(CO) 6 , V(C 5 H 5 ) 2 , and H 2 Se, respectively. [8,18] The introduction of vanadium atoms has a direct impact on the surface morphology of the obtained films, which are fully coalesced and primarily monolayer with some secondary-islands on top. (Figure 1a,b).…”

“…[27] Both intrinsic and extrinsic FEOL films exhibit characteristic W 4f 7/2 , W 4f 5/2 , and W 5p 3/2 peaks at 32.6, 34.8, and 38.1 eV and Se 3d 5/2 and Se 3d 3/2 peaks at 54.8 and 55.7 eV, respectively (Figure 1c and Figure S4a, Supporting Information) agreeing with previous reports. [18,28] As the vanadium concentration increases, associated V 2p 3/2 and V 2p 1/2 XPS peaks are detectable at 513.7 and 521.7 eV, respectively, corresponding to VSe bonding and indicating successful doping of the WSe 2 with V atoms (Figure 1d and Figure S3b, Supporting Information). [29] Moreover, a peak at ≈517.0 eV is observed that corresponds to VO bonding, and suggests vulnerability of extrinsic films toward oxidation (Figure 1d and Figure S4b, Supporting Information).…”

“…To date, wafer-scale synthesis of near electronic-grade intrinsic TMDCs has been successfully realized at FEOL (>700 °C) [8][9][10] and BEOL(<500 °C) [11][12][13][14] compatible temperatures. However, while various doping techniques can tune the electrical conductivity of TMDCs (e.g., surface charge transfer doping, [15] electrostatic doping, [16] intercalation, [17] and substitutional doping, [18,19] ) progress is still limited in truly scalable doping. Furthermore, even though "proof-of-concept" devices have been demonstrated based on doped TMDCs including vanadium dopants, [20][21][22][23][24] uniform distribution and precise control of the impurity density over a large scale and use of methods compatible with the state-of-the-art Si CMOS 300 mm process lines, still remains challenging.…”

Controllable Thin‐Film Approaches for Doping and Alloying Transition Metal Dichalcogenides Monolayers

“…d) Illustration of the vertical, cold-wall MOCVD system that is used to grow Re-oped WSe 2 samples. Reproduced with permission [96. Copyright 2020, Wiley-VCH.e) Schematic of the multi-step process showing variation in W(CO) 6 flow rate that was used to control nucleation, ripening, and lateral growth.…”