By Nidhi DhullReviewed by Lexie CornerNov 14 2024
A recent article in Nature Communications introduced a method for precise p- and n-type substitutional doping of two-dimensional (2D) semiconductors. This method was applied for the one-step growth of spatially selective and precisely doped 2H-MoTe2 thin films. These films were then used to fabricate a chip-sized 2D complementary metal-oxide-semiconductor (CMOS) inverter array.
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Background
Doping is an essential process in the fabrication of semiconductor devices. For example, doping silicon with phosphorus or boron during melting produces single-crystal silicon wafers with different conductivity types and levels. The ability to precisely dope p- and n-type regions through ion implantation is also important for adjusting the threshold voltage of CMOS field-effect transistors (FETs).
2D semiconductors, which are atomically thin and have stacking potential, offer opportunities for advancing semiconductor technology. However, using 2D semiconductors in large-scale integrated circuits requires precise control over the fabrication of patterned p- and n-type channels with accurate doping.
Traditional techniques, such as ion implantation, are not applicable to 2D semiconductors, and a general method for controllable p- and n-type doping in these materials has not yet been established. This study introduces a method for synthesizing 2H-MoTe2 with spatially patternable and precisely controlled dopant types and concentrations through substitutional doping.
Methods
Nb-doped or Re-doped 2H-MoTe2 films were grown on 1-inch Si/SiO2 substrates via magnetron sputtering (Mo incorporated with Nb or Re films) and chemical vapor deposition (Nb-doped or Re-doped 2H-MoTe2). The Nb incorporation ratios in 2H-MoTe2 films were 0.10 %, 0.19 %, 0.20 %, and 0.27 %, while the Re incorporation ratios were 0.06 %, 0.09 %, 0.24 %, and 0.42 %.
Device fabrication
Back-gate devices were fabricated using Nb-doped or Re-doped 2H-MoTe2 thin films (~5 nm). The channels were defined through photolithography and reactive ion etching. Pd/Au (10/30 nm) electrodes were used to contact the 2H-MoTe2 channel, while Bi/Au (10/30 nm) electrodes were used for the 2H-MoTe2 channel. Hall devices were fabricated using the same process. All devices were then covered with a 20-nm Al2O3 layer deposited by atomic layer deposition to protect them from exposure to air.
CMOS inverters were fabricated on spatially patterned doped 2H-MoTe2 films using the same procedure. However, Ti/Au (10/30 nm) electrodes were used to contact the n-type channel as the CMOS devices underwent rapid thermal annealing (RTA) at 300 °C for 30 seconds to eliminate the trap states in the Al2O3 dielectric.
The devices were characterized using Raman spectroscopy, atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), and scanning transmission electron microscopy (STEM). The electrical properties of the transistors and Hall devices were measured using a semiconductor characterization system and a source meter, respectively.
Results and Discussion
The Hall measurements of the Nb- and Re-doped samples showed opposite polarities, confirming that the Nb-doped 2H-MoTe2 films contained hole carriers, while the Re-doped films contained electron carriers. The unintentionally doped 2H-MoTe2 had a low background hole concentration, indicating the high quality of the film synthesized through 2D epitaxy growth.
Adding 0.06 % Re to the 2H-MoTe2 film made it an electron-conducting material with doping resolution as low as approximately 1010 cm⁻². The electron concentration increased with higher levels of Re doping, while the hole concentration increased with higher levels of Nb doping. As a result, the conductivity of the doped 2H-MoTe2 films gradually increased with higher carrier concentrations for both p- and n-type doping.
The Nb-doped 2H-MoTe2 demonstrated p-type semiconductor behavior, with conductance increasing as doping concentration increased. In contrast, the Re-doped 2H-MoTe2 exhibited ambipolar transport behavior at low doping levels (0.06 %) and n-type semiconductor transport characteristics at higher doping concentrations.
KPFM measurements showed that the Re-doped 2H-MoTe2 had a significantly higher surface potential than the Nb-doped 2H-MoTe2, consistent with the expected doping characteristics. Additionally, arbitrarily patterned in-plane coplanar-contacted p- and n-type 2H-MoTe2 films demonstrated the reproducibility and design flexibility of the one-step growth method.
The compatibility of patterned doped 2H-MoTe2 thin films with large-scale fabrication was demonstrated through the successful production of hundreds of CMOS inverter arrays on a centimeter-scale chip. The transport characteristics of these devices matched those of p- and n-FETs, confirming well-controlled doping concentrations in the p- and n-MoTe2 channels.
Conclusion
The researchers successfully demonstrated a substitutional doping method that enables the direct synthesis of large-area 2D semiconductor 2H-MoTe2 thin films with precisely controlled doping, allowing for the fabrication of p-type, n-type, or pn-patterned structures.
The stability of the doping technique and uniformity of the electrical performance of the doped films were confirmed by the transfer characteristics of 30 p-type and 30 n-type 2H-MoTe2 transistors.
In addition to its potential for large-scale production, the in-plane 2D epitaxy of the doped 2H-MoTe2 films allows for their integration with various surfaces, providing new functionalities for silicon chips. Furthermore, these p- and n-type 2D semiconductor channels can be grown either in-plane or layer-by-layer in the vertical direction, enabling the development of new interlayer interconnection processes.
Journal Reference
Pan, Y., et al. (2024). Precise p-type and n-type doping of two-dimensional semiconductors for monolithic integrated circuits. Nature Communications. DOI: 10.1038/s41467-024-54050-2, https://www.nature.com/articles/s41467-024-54050-2
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