Surface Charge Transfer Doping of III−V Nanostructures

Surface charge transfer is presented as an effective doping technique for III−V nanostructures. We generalize that the technique is applicable to nanoscale semiconductors in the limit where carriers are quantum confined. As a proof-of-concept, potassium surface charge transfer doping is carried out for one-dimensional (1D) and two-dimensional (2D) InAs on Si/SiO2 substrates. Experiments and simulations show that equivalent dopant areal dose of up to ∼2 × 10 cm−2 is obtained, which is sufficient for degenerate doping of InAs nanostructures. This work presents a new pathway for controllable doping of inorganic semiconductors with limits fundamentally different from those of substitutional doping. I recent years, III−V semiconductors have been intensively explored for future energy efficient electronics, in part due to their high mobility and saturation velocity as compared to Si. For planar field-effect transistors (FETs), a general rule of thumb dictates that to obtain effective gate electrostatic coupling of the channel, a body (i.e., channel) thickness of <1/ 3 of the channel length should be used to minimize the short channel effects. This suggests that for sub-10 nm channel length devices, the semiconductor thickness should be <3 nm. At this thickness, the carriers in most semiconductors are heavily quantized by the structural confinement effects. This is particularly true for III−V compound semiconductors, which have large excitonic Bohr radii (e.g., InAs Bohr radius is aBohr ≈ 32 nm). At this extreme scaling limit, new challenges and opportunities arise given the strong quantum confinement of carriers and the molecular-scale dimensions (at least in thickness) of the semiconductors. An example includes uniform and controllable doping. When the thickness is reduced to sub 3-nm, the number of dopant atoms needed per unit area even for high doping concentrations becomes very small, thereby causing large stochastic variation. More importantly, ion implantation is not compatible with nanoscale III−V compound semiconductors because it causes severe structural damage that cannot be perfectly fixed by a subsequent thermal anneal. To address this challenge, new doping techniques are needed. In one approach, we recently demonstrated monolayer doping (MLD) of semiconductors as a platform for enabling nanoscale substitutional doping in Si and III− V structures. The concept relies on the formation of selflimiting molecular monolayers containing dopant species on the surface of semiconductors first, followed by their diffusion into the lattice by a subsequent thermal annealing process. MLD has been shown to be highly versatile and promising as compared to conventional ion implantation doping, enabling extremely shallow doping profiles, down to a few nanometers without structural defects. However, although better than ion implantation, this technique is still based on substitutional doping and diffusion of dopants, and hence at the ultrascaled miniaturization limit, it also suffers from stochastic variation. Previously, surface charge transfer doping of molecular systems, such as polymers, carbon nanotubes, graphene, and layered chalcogenides, was demonstrated as an effective path for controlling the carrier concentrations. As a guideline, doping occurs when the surface dopant, which is often an atom or a molecule, is preferentially oxidized (n-doping) or reduced (p-doping) by the semiconductor. Specifically, n-doping occurs when the highest occupied orbital is above both (i) the lowest conduction subband edge and (ii) the Fermi level in the semiconductor, and p-doping occurs when the lowest unoccupied orbital is both (i) below the highest valence subband edge and (ii) the Fermi level in the semiconductor. However, attempts to use surface charge transfer doping with conventional inorganic semiconductors have largely failed, or resulted in only minimal observed effects. Here, by using InAs nanomembranes (2D) and nanowires (1D) as prototypical material systems, we demonstrate for the first time that surface charge transfer doping can also be highly effective in conventional inorganic semiconductors within a certain constraint. Specifically, as long as the carriers are structurally quantum confined, strong modulation of carrier concentration Received: June 22, 2013 Revised: July 30, 2013 Published: July 31, 2013 Article