Atomically Precise Placement of Single Dopants in Si

Abstract

We demonstrate the controlled incorporation of P dopant atoms in Si (001) presenting a new path toward the creation of atomic-scale electronic devices. We present a detailed study of the interaction of PH3 with Si (001) and show that it is possible to thermally incorporate P atoms into Si (001) below the H desorption temperature. Control over the precise spatial location at which P atoms are incorporated was achieved using STM H-lithography. We demonstrate the positioning of single P atoms in Si with ∼ 1 nm accuracy and the creation of nanometer wide lines of incorporated P atoms.PACS numbers: 03.67. Lx, 68.37.Ef, The ability to control the location of individual dopant atoms within a semiconductor has enormous potential for the creation of atomic-scale electronic devices, including recent proposals for quantum cellular automata [1], single electron transistors [2] and solid-state quantum computers [3]. Current techniques for controlling the spatial extent of dopant atoms in Si rely on either ion implantation techniques, or dopant diffusion through optical or electron-beam patterned mask layers. While the resolution of these techniques continues to improve they have inherent resolution limits as we approach the atomicscale [4]. The work presented here looks beyond conventional techniques to position P dopant atoms with atomic-precision by using scanning tunneling microscopy (STM) based lithography on H passivated Si (001) surfaces [5,6] to control the adsorption and subsequent incorporation of single P dopant atoms into the Si (001) surface.First, we show the controlled adsorption of PH 3 molecules to STM-patterned areas of H-terminated Si (001) surfaces. In these studies, we have used the H-terminated surface as a reference where the intrinsic surface periodicity can be observed to identify both adsorbed PH 3 molecules [7] and the previously unobserved room temperature dissociation product, PH 2 . We then show, using low PH 3 dosed clean Si (001) surfaces, that both of these room temperature adsorbates can be completely dissociated using a critical anneal, and more importantly, that this results in the substitutional incorporation of individual P atoms into the top layer of the substrate. Finally, we combine these two results to demonstrate the spatially controlled incorporation of individual P dopant atoms into the Si (001) surface with atomicscale precision. Of crucial importance to this final result is that the anneal temperature for P atom incorporation lies below the H-desorption temperature, so that the Hresist layer effectively blocks any surface diffusion of P atoms before their incorporation into the substrate surface.Figures 1(a) -1(c) demonstrate the flexibility of STM H-lithography to create different sized regions of bare Si (001) surface. As we will show, these regions can be used not only as a template for dopant incorporation but also to aid in fundamental studies of surface reactions. Figures 1(a) and 1(b) show the creation of both large areas (200 × 30 nm 2 ) and parallel, nanometer-wide lines...

Silicon-based spin and charge quantum computation

“…From the perspective of current QC fabrication efforts, ∼ 1 nm accuracy in single P atom positioning has been recently demonstrated (Schofield et al 2003), representing a major step towards the goal of obtaining a regular donor array embedded in single crystal Si. Exchange coupling distributions consistent with such accuracy are presented by , indicating that even such small deviations (∼ 1 nm) in the relative position of donor pairs can still lead to significant changes in the exchange coupling, favoring J ∼ 0 values.…”

“…are required for a universal QC, involve precise control over electron-electron exchange (Loss and DiVincenzo 1998, Kane 1998, Vrijen et al 2000, Hu and Das Sarma 2000. Such control can presumably be achieved by fabrication of donor arrays with well-controlled positioning and surface gate potential (O'Brien et al 2001, Schofield et al 2003, Buehler et al 2002, Schenkel et al 2003. However, electron exchange in bulk silicon has spatial oscillations (Andres et al 1981, Koiller et al 2002 on the atomic scale due to valley interference arising from the particular six-fold degeneracy of the bulk Si conduction band.…”

“…7. We incorporate the effect of small uncertainties by taking R u = 1nm, corresponding to the best reported degree of accuracy in single P atom positioning in Si (Schofield et al 2003). These small deviations completely change the qubit gap distribution, as given by the histogram in Fig.…”

Field-directed sputter sharpening for tailored probe materials and atomic-scale lithography

“…S canning tunnelling microscopy (STM) facilitates atomic-scale lithography by hydrogen resist patterning; the technique is especially useful for fundamental processes such as patterned oxidation 1 , metallization 2,3 , dopant incorporation 4 , templating of organic molecules 5,6 and fabrication of quantum cellular automata devices 7 , all at the atomic-scale. Sharpening of probes further extends to fi eld emitters 8 in displays and electron microscopes as well as razor blade manufacturing 9 .…”

Robust controlled-NOT gate in the presence of large fabrication-induced variations of the exchange interaction strength

“…The cause of this is the inter-valley interference between the six degenerate conduction band minima of silicon, resulting in oscillations of the exchange coupling strength 3,4,5,6 . Exact positioning of donors to better than 2-3 sites is difficult 7 and therefore we expect significant uncertainty in the un-biased strength of the coupling between donors. The uncertainty in our knowledge of the coupling, leads to error in gate operation.…”