Figure 1: STM images of the Si(001) surface after exposure to phosphine gas. The arrows indicate four species formed as a result of phosphine dissociation that were identified in our studies.	Figure 2: (a) Filled state STM image of a hydrogen terminated Si(001) surface after it has been patterned with the STM to form the letters CQCT and (b) STS measurements of the same surface after it has been phosphine dosed and encapsulated in silicon, highlighting the integrity of the buried dopants after encapsulation.

Figure 3: SEM images, schematic diagrams and data for Si:P nanoMOSFET and Si:P metallic dot with source-drain leads, respectively. (c) Periodic charging of the central dot for device in (b), detected using an rf-SET.

Bottom-up Atomic Assembly of Si:P Qubits
During 2003 we demonstrated the ability to insert individual P atoms into a Si(001) surface with atomic precision and fabricated one of the first devices ever to be made by STM (scanning tunnelling microscope) patterning. In 2004 we have built on these successes and, with the aim of controllably incorporating P atoms for qubit scale-up, now understand the detailed surface chemistry of the phosphine gas PH3 – Si(001) surface adsorption system. We performed exhaustive experimental studies of the phosphine doped silicon surface, see Figure 1. A detailed first-principles survey of all conceivable dissociation products has allowed us to identify all the species on the surface and produce a reaction pathway to P incorporation in silicon.

Once an array of P atoms has been formed on the surface it is important to image the P dopants beneath the Si(100) surface after encapsulation. Such buried-dopant imaging can confirm that the integrity of the array has not been compromised during MBE overgrowth. Figure 2(a) is a filled state image of a H-terminated Si surface that has been patterned to say ‘CQCT’. After PH3 dosing, annealing and encapsulation in silicon the same surface is shown in Figure 2(b). Under normal scanning conditions it is difficult to observe the STM patterned array under this surface. As such we have taken spectroscopic I-V curves at each point in the topographic image, a technique called scanning tunnelling spectroscopy which allows us to identify the unique signature of buried phosphorus dopants.

By developing a registration technique last year, we were able to overcome the difficult challenge of making electrical contact to STM-patterned buried dopant layers once the chip was removed from the STM. Over the past year we have now fabricated the narrowest phosphorus-doped conducting wires in silicon. We find that with widths below 25nm the wires stop conducting and detailed investigations are underway to understand this.

Silicon-based Si:P Qubits via Ion Implantation
In 2003 Centre researchers constructed and demonstrated a double quantum dot structure, implanted with ~600 phosphorus atoms in silicon using phosphorus ion implantation. During 2004 the teams at UNSW and the University of Melbourne significantly advanced the single-ion detection capability to unambiguously register a single phosphorus ion impact with near 100% confidence. With these detectors it is now possible to configure a device with a precisely counted number of implanted P atoms.

Over the past year a wide variety of ion-implanted structures were measured using both dc and rf SETs (single electron transistors). Nanostructures have been designed to study charge transport and disorder effects in the implanted Si:P environment. A combination of direct current (I-V) measurements and simultaneous indirect charge sensing using a surface SET can yield information about the nature of defects in the barrier between locally doped (implanted) regions. Figure 3(a) depicts such a ‘nanoMOSFET’ device and shows data in which Coulomb blockade is observed in the I-V characteristic, indicative of charging in small ‘puddles’ created by stray dopants or defects in the nanoMOSFET channel.

In Figure 3(b), a single implanted P-cluster with source-drain leads allows direct transport measurements to be correlated with remote charge detection using a surface SET. In the low conduction regime the highly sensitive SET detection enables studies of single electron tunnelling on the cluster and barrier control. When operated at radio frequencies using our rf-SET capability we observe a very clear sawtooth potential on the central dot, with random occupancy of non-equilibrium charge states which increases with temperature – see Figure 3(c).

A prerequisite for the coherent manipulation of a Si:P system is the application of gate signals on time scales significantly faster than the qubit dephasing time. During 2004 we developed a cryogenic platform, combining ultra-fast gate signals with our existing twin rf-SET charge detection capability. This has been very successful and we have now been able to use microwave signals (~40 GHz) to coherently drive transitions between the two basis states |0> and |1> under continuous measurement with the SET.

Robert Clark, Michelle Simmons
and Alex Hamilton