Charge sharing in multi-electrode devices for deterministic doping studied by IBIC

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Abstract

Following a single ion strike in a semiconductor device the induced charge distribution changes rapidly with time and space. This phenomenon has important applications to the sensing of ionizing radiation with applications as diverse as deterministic doping in semiconductor devices to radiation dosimetry. We have developed a new method for the investigation of this phenomenon by using a nuclear microprobe and the technique of Ion Beam Induced Charge (IBIC) applied to a specially configured sub-100 μm scale silicon device fitted with two independent surface electrodes coupled to independent data acquisition systems. The separation between the electrodes is comparable to the range of the 2 MeV He ions used in our experiments. This system allows us to integrate the total charge induced in the device by summing the signals from the independent electrodes and to measure the sharing of charge between the electrodes as a function of the ion strike location as a nuclear microprobe beam is scanned over the sensitive region of the device. It was found that for a given ion strike location the charge sharing between the electrodes allowed the beam-strike location to be determined to higher precision than the probe resolution. This result has potential application to the development of a deterministic doping technique where counted ion implantation is used to fabricate devices that exploit the quantum mechanical attributes of the implanted ions.

Introduction

Fifty years ago the metastable energy levels in Cr doped Al2O3 were used to create the required population inversion for the first laser [1]. Associated with this development was a widespread search for other long-lived excited states in the solid state. The exceptionally long relaxation time of electron spins in P doped Si were identified as specially promising [2], [3]. This long relaxation time is now recognised as having potential applications in solid state quantum information processing devices [4], [5] and successful coherent control of electronic states in this system has recently been demonstrated [6]. Fabrication of useful devices depends on (i) the development of single atom deterministic doping techniques which are now emerging [7], [8], [9] and (ii) techniques to probe and control the quantum states of single atoms which have now been demonstrated [10], [11], [12]. Deterministic doping by ion implantation has been used to configure a two P atom device in which the charge state of the individual donors was controlled [13]. Even more promising is the successful demonstration [14] of control and readout of a single electron spin on an implanted P atom which shows the immense potential of this class of devices.

The deterministic doping method for these devices relies on the detection of the charge transients induced on surface electrodes by single ion impacts where the incident ion kinetic energy is partially dissipated by electronic stopping processes that ionise the substrate. The aim of the present work is to investigate the diffusion and drift of the induced charge by means of two separated electrodes and the use of a nuclear microprobe to control the ion strike location relative to the electrode positions. This will allow the proportion of charge shared between the electrodes to be used to estimate the ion strike location independent of knowledge of the position of the beam. This method offers several advantages over traditional silicon position sensitive detectors [15] because the substrate does not have to be doped or subject to damage gradients [16] and higher spatial resolution is possible compared to silicon strip detectors [17] where new designs [18] offer resolution down to 25 μm. In the present experiments the electrode separation was the same order of magnitude as the ion range and the microbeam resolution was about an order of magnitude smaller.

Section snippets

Experiment

The details of our device and the experimental configuration have been previously described [7]. Briefly, we employ a p-i-n structure fabricated in a high-purity silicon substrate (>18,000 Ωcm) [19] configured with two independent front surface aluminium detector electrodes which make contact with two boron-doped p-wells (∼1020 cm−3) (see schematic in Fig. 1 and images in Fig. 3a,b). These electrodes will be termed “L” and “R” hereafter. A further n-type back contact is fabricated from a

Results

Two separate scans of the device are presented here. In the first scan, a single-station IBIC map of the device was obtained from the L and R electrodes connected together as shown in Fig 3c. This map reveals the highest charge collection efficiency is from the thin oxide region of the device as expected and the associated energy spectrum (not shown) has a Full Width at Half Maximum (FWHM) of 15 keV which is the system noise corresponding to the electronics (in this and subsequent measurements

Conclusion

We have demonstrated that it is possible to employ the independent IBIC signals from surface detector electrodes to measure the ion strike location in a silicon device. For MeV ions incident on a suitably configured device, sub-micron precision is possible. Configuration of the device with an orthogonal third electrode could potentially allow maps to be made with an unfocused beam to sub-micron precision. By scaling the system down and employing sub-15 keV ions for sub-20 nm deep implants of

Acknowledgements

This work was funded by the Australian Research Council Centre of Excellence scheme and the US Army Research Office under Contract No. W911NF-08-1-0527. The authors are grateful for technical support from Roland Szymanski and Alberto Cimmino.

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