Towards the fabrication of phosphorus qubits for a silicon quantum computer

Abstract

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...

Atomically Precise Placement of Single Dopants in Si

“…Using both higher PH 3 dose rates (10 −8 mbar chamber pressure) and total doses (∼ 3 L), we have noted the adsorption of PH 3 at single H desorption sites [21]. However, with the low dose rate and fluence used here (10 −9 mbar and 0.3 L) we have observed an almost complete absence of adsorption at single H desorption sites, with coverages approaching saturation in the larger lithographically defined desorption regions.…”

Split-off dimer defects on theSi(001)2×1surface

“…There are currently several exciting proposals to use the (001) surface of silicon for the construction of atomicscale electronic devices, including single electron transistors 1 , ultra-dense memories 2 and quantum computers 3,4 . However, since any random charge or spin defects in the vicinity of these devices could potentially destroy their operation, a thorough understanding of the nature of crystalline defects on this surface is essential.…”

High mobility two-dimensional electron system on hydrogen-passivated silicon(111) surfaces

“…The Si(111) surface is thus unique in that such a simple wet chemical treatment can produce an ideal H-passivated Si surface which is atomically flat and defect free [3]. In addition, the H-Si(100) surface has been utilized recently as a resist for controlling the placement of individual atoms [4,5]. Unfortunately H-passivated surfaces are sensitive to ambient conditions (oxidizes ∼ hours) and elevated temperatures (T ≥ 300 • C).…”