Review article
Deterministic doping

https://doi.org/10.1016/j.mssp.2016.10.039Get rights and content

Abstract

Emerging programs in a new field of technology that employs quantum mechanical principles in engineered devices has driven new approaches to atomic-scale fabrication. Of crucial importance is the capability to configure single atoms in silicon, diamond and other materials. These engineered materials form the foundations of quantum technology which includes the fields of quantum communication and quantum computing. Quantum technology exploits quantum superposition and entanglement in potentially scalable quantum devices. To insert donor atoms in a large-scale device methods for deterministic ion implantation have been developed. These methods potentially allow the standard techniques developed for engineering materials for the Information Technology industry to be employed to make devices that exploit the new technologies. This paper reviews the emerging new technologies for deterministic doping to address the challenges of engineering atoms in the solid state.

Introduction

The second quantum revolution, as described by Dowling [1], is the development of new technologies employing quantum mechanics that has now emerged as a significant field of research. These technologies include quantum computing [2], [3], [4], quantum cryptography [5], quantum simulation [6], [7], quantum metrology [8], [9], quantum sensing in biology [10] or geology [11], quantum time keeping [12], quantum imaging [13] and the quantum internet [14]. There has been significant progress in the development of devices based on the new field of silicon quantum electronics [15] or photonics [16]. In the case of solid state devices employing nuclear or electron spins [17] based on silicon [18] or diamond [19] large scale devices will require the precision engineering of single dopant atoms or colour centres in a crystalline matrix. Ion implantation is already highly developed for silicon in the information technology industry [20] and significant progress has been made towards employing ion implantation to build devices engineered with single atoms in silicon and other materials [21]. For the fabrication of arrays of one or more single atoms the technique of deterministic ion implantation has been developed where implantation is done in conjunction with a method for the registration of each implanted ion.

Recent demonstrations of electron and nuclear spin control of single phosphorus dopant atoms implanted into silicon have shown single donor spin qubits in silicon are strong contenders in the quest for the realization of a scalable quantum computer device [22], [23], [24], [25]. The next step of demonstrating reliable two qubit coupling is likely achievable with existing technology but for this to be achieved on a larger scale requires new techniques for building atomic arrays of implanted donors into silicon including deterministic ion implantation. Also, although much of the present focus is on phosphorus, the heavy donor atoms of arsenic, antimony and bismuth can be implanted into a silicon lattice at a given depth with higher spatial resolution due their higher mass (see Table 1). Also, in the case of arsenic and antimony, diffusion in silicon is less affected by oxidation enhanced diffusion [28] and hence potentially offers higher placement precision compared to phosphorus considering the need for post-implant annealing steps. In the case of antimony the donor electron Bohr orbit diameter is larger than the other donors [26] which may serve to relax constraints on construction precision. Implantation of heavy donor atoms into silicon will cause significant lattice damage, but it has been shown that an ultra-scaled CMOS device can withstand the implantation of erbium isotopes (161<A<171, comparable to bismuth: A=209) where the optically excited charge and spin state of a single implanted erbium atom was detected electrically [29]. Also bismuth donor electron spin resonance can be successfully measured in ensembles implanted into silicon [30]. A potential new application beyond spin qubits in the case of bismuth donors in silicon arises from the fact that the donor electron exhibits technologically useful clock transitions which are insensitive to external perturbations [31]. This transition has so far only been studied in ensembles not in single donors. This demonstrates that ion implantation into wafer scale arrays of fully fabricated and encapsulated devices is a robust method for the introduction of single and few ions and it can be expected that this will also be true for dopant arrays.

Section snippets

Materials and methods

By placing single arsenic, antimony and bismuth atoms in arrays, it is possible, at low temperatures, to configure the local electrostatic landscape of the device for manipulation of the associated donor electrons and through spin-orbit coupling, also the donor nuclear spins. Magnetoelectric transport through single donors is affected by the donor electron level structure, which depends on the dopant-specific properties. Some of these applications employ standard analytical spectroscopic

Silicon

The experimental results from a production run to fabricate counted two or more atom devices with this method is shown in Fig. 3. Here the surface mask contained two 15 nm diameter nanoapertures separated by 50 nm irradiated by a 14 keV P ion beam. The ions can only enter the substrate through the apertures and hence the irradiation continued until two ion implantation signals were recorded above the noise threshold of the on-chip detector electrodes and the associated charge-sensitive electronics.

Conclusion

Ion implanted silicon devices have demonstrated a remarkable range of controlled quantum phenomena, including a recent test of Bell's inequality [78] utilising the electron and nuclear spins of a single implanted 31P atom. Although there are numerous technical challenges to building a large scale device [79], [80], [81] with an array of deterministically implanted atoms, it is probable there will be a convergence of the localisation precision that can be achieved for sub-20 nm range ion

Acknowledgments

This research was supported by the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (project number CE110001027) and the US Army Research Office (grant number W911NF-08-1-0527). We acknowledge useful discussions with Thomas Schenkel, Ettore Vittone, Jan Meijer, Daniel Spemann, Kumar Ganesan, Daniel Creedon, Steven Prawer, Andrew Dzurak, Fay Hudson and Andrea Morello. We also acknowledge past members of our group for their contributions to

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