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...

Individual atoms and ions are now routinely manipulated using scanning tunnelling microscopes or electromagnetic traps for the creation and control of artificial quantum states. For applications such as quantum information processing, the ability to introduce multiple atomic-scale defects deterministically in a semiconductor is highly desirable. Here we use a scanning tunnelling microscope to fabricate interacting chains of dangling bond defects on the hydrogen-passivated silicon (001) surface. We image both the ground-state and the excited-state probability distributions of the resulting artificial molecular orbitals, using the scanning tunnelling microscope tip bias and tip-sample separation as gates to control which states contribute to the image. Our results demonstrate that atomically precise quantum states can be fabricated on silicon, and suggest a general model of quantum-state fabrication using other chemically passivated semiconductor surfaces where single-atom depassivation can be achieved using scanning tunnelling microscopy.

The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices however requires atomic scale manipulation -an immense technological challenge. We demonstrate that it is possible to fabricate an atomically-precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a siliconbased quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth. (To appear in Phys. Rev. B Rapid Comm.) 03.67. Lx, 68.37.Ef, A quantum bit (or qubit) is a two level quantum system that is the building block of a quantum computer. To date the most advanced realisations of a quantum computer are qubit ion trap 1 and nuclear magnetic resonance 2-4 systems. However scaling these systems to large numbers of qubits will be difficult 5 , making solidstate architectures 6 , with their promise of scalability, important. In 1998 Kane proposed a novel solid state quantum computer design 7 using phosphorus 31 P nuclei (nuclear spin I = 1/2) as the qubits in isotopically-pure silicon 28 Si (I = 0). The device architecture is shown in Fig. 1a, with phosphorus qubits embedded in silicon approximately 20 nm apart. This separation allows the donor electron wavefunctions to overlap, whilst an insulating barrier isolates them from the surface control A and J gates. These A and J gates control the hyperfine interaction between the nuclear and electron spins and the coupling between adjacent donor electrons respectively. For a detailed description of the computer operation refer to Kane 7 . An alternative strategy using the electron spins of the phosphorus donors as qubits has also been proposed 8 .One of the major challenges of this design is to reliably fabricate an atomically-precise array of phosphorus nuclei in silicon -a feat that has yet to be achieved in a semiconductor system. Whilst a scanning tunnelling microscope (STM) tip has been used for atomic scale arrangement of metal atoms on metal surfaces 9 , rearrangement of individual atoms in a semiconductor system is not straightforward due to the strong covalent bonds involved. As a result, we have employed a hydrogen resist strategy outlined in Fig. 1b. Here the array is fabricated using a resist technology, much like in conventional lithography, where the resist is a layer of hydrogen atoms that terminate the silicon surface. An STM tip is used to selectively desorb individual hydrogen atoms, exposing the underlying silicon surface in the required array. STM induced hydrogen desorption has been developed and refined over the past ten years 10 and has been proposed 11 for the assembly of atomically-ordered device structure...