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Monolayer doping
Monolayer doping (MLD) is a well controlled, wafer-scale surface doping technique first developed at the University of California, Berkeley, in 2007.〔 This work is aimed for attaining controlled doping of semiconductor materials with atomic accuracy, especially at nanoscale, which is not easily obtained by other existing technologies. This technique is currently used for fabricating ultrashallow junctions (USJs) as the heavily doped source/drain (S/D) contacts of metal-oxide-semiconductor field effect transistors (MOSFETs) as well as enabling dopant profiling of nanostructures.
This MLD technique utilizes the crystalline nature of semiconductors and its self-limiting surface reaction properties to form highly uniform, self-assembled, covalently bonded dopant-containing monolayers followed by a subsequent annealing step for the incorporation and diffusion of dopants.〔J. C. Ho, R. Yerushalmi, Z. A. Jacobson, Z. Fan, R. L. Alley, A. Javey, "Controlled nanoscale doping of semiconductors via molecular monolayers", ''Nature Materials'', 7 (1), 62-67, 2008.〕 The monolayer formation reaction is self-limiting, thereby, resulting in the deterministic coverage of dopant atoms on the surface. MLD differs from other conventional doping techniques such as spin-on-dopants (SODs) and gas phase doping techniques in the way of dopant dose control. Such control in MLD is much more precise due to the self-limiting formation of covalently attached dopants on the surface while the SODs just rely on the thickness control of the spin-on oxide and the gas phase technique depends on the control of dopant gas flow rate; therefore, the excellent dose control in MLD can yield the exact tuning of the resulting dopant profile. Compared to ion-implantation, MLD does not involve the energetic introduction of dopant species into the semiconductor lattice where crystal damages are induced. In the case of implantation, defects such as interstitials and vacancies are inevitably generated, which interact with the dopants to further broaden the junction profile. This is known as the transient-enhanced diffusion (TED), which limits the formation of good quality of USJs. Also, stochastic variation in the dopant positioning and sever stoichiometric imbalance are thus induced for binary and tertiary compound semiconductors by the implantation techniques. In contrast, all MLD dopant atoms are thermally diffused from the crystal surface to the bulk and the dopant profile can be easily controlled by the thermal budget. Since the MLD system can be classified as a limited source model, this is desirable for controlled USJ fabrication with high uniformity and low stochastic variation. Combined with the excellent dopant dose uniformity and coverage in MLD, it is especially attractive for doping nonplanar devices such as fin-FETs and nanowires. As a result, high quality sub-5 nm ultra-shallow junction has been demonstrated in silicon via the use of this MLD technique.〔
==Applications in various structures==
The MLD process is applicable for both p- and n-doping of various nanostructured materials, including conventional planar substrates, nanobelts and nanowires, which are fabricated by either the ‘bottom-up’ or ‘top-down’ approaches, making it highly versatile for various applications. In p-type doping of silicon, a covalently anchored monolayer of allylboronic acid pinacol ester is formed on the surface as the boron precursor while a monolayer of diethyl 1-propylphosphonate is used as the phosphorus precursor in n-type doping.〔 For example, in the case of USJ formation, combining the phosphorus-MLD and conventional spike annealing, the record 5 nm junction (down to 2 nm - the SIMS resolution limit) with the noncontact Rs measurements (~5000 Ω/□) is reported and being consistent with the predicted values from the dopant profile.〔J. C. Ho, R. Yerushalmi, G. Smith, P. Majhi, J. Bennett, J. Halim, V. Faifer, A. Javey. "Wafer-Scale, Sub-5 nm Junction Formation by Monolayer Doping and Conventional Spike Annealing", ''Nano Letters'', 9 (2), 725–730, 2009.〕 Notably, ~70 % of the dopants are electrically active as the MLD process utilizes an equilibrium based diffusion mechanism.〔
In addition to silicon, MLD has also been applied to compound semiconductors such as indium arsenide (InAs) to obtain high quality ultra-shallow junctions. For the past years, controlling the post-growth dopant profiles in compound semiconductors such as III-V materials deterministically has not been well achieved due to the challenges in controlling the recovered stoichiometry after the implantation and sequential annealing. These residual damages can lead to higher junction leakage and lower dopant activation in compound semiconductors. Utilizing the MLD technique with sulfur dopants, a dopant profile abruptness of ~ 3.5 nm/decade with high electrically active sulfur concentrations of ~ 8–1018 cm−3 is observed in InAs without significant defect density.〔J. C. Ho, A. C. Ford, Y.-L. Chueh, P. Leu, O. Ergen, K. Takei, G. Smith, P. Majhi, J. Bennett, A. Javey. "Nanoscale doping of InAs via sulfur monolayers", ''Applied Physics Letters'', 95, 072108, 2009.〕 Remarkably. the MLD capping layer serves as i) preventing group V elements to desorb and ii) avoiding the dopant atoms to be lost to the ambient in order to result in the good quality junctions. All these can further demonstrates the utility of this innovative approach for device fabrication.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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