Activation and electron spin resonance of near-surface implanted bismuth donors in silicon

The bismuth (Bi) substitutional donor in silicon (Si) is an attractive qubit candidate for quantum computing proposals due to its large Hilbert space, clock transitions, and potential to couple to superconducting flux qubits. Single-qubit control, coupling, and readout by surface nanocircuitry requires a Bi depth of $\sim 20$ nm in Si. This can be achieved using ion implantation of $\sim 25$ keV Bi. This work explores the activation properties of Bi implanted at 26 keV with fluences of $1\times {10}^{14}$ and $6\times {10}^{12}{\mathrm{cm}}^{-2}$ into both crystalline and preamorphized Si. The Bi electrical activation yield was measured over a broad range of annealing conditions using resistivity and Hall effect measurements, enabling optimal annealing strategies to be proposed for the different implant parameters. For the high and low fluences, the maximum Bi activation yields achieved were 64% and 46%, respectively. Above a critical thermal budget, a substantial fraction of Bi forms electrically inactive complexes in the high fluence sample only. The substitutional fraction and diffusion of high fluence Bi was quantified, with diffusion coefficients $D_0=4.0\pm 0.5$ and $7.5\pm 0.5{\mathrm{cm}}^2{\mathrm{s}}^{-1}$ found for implantation into crystalline and preamorphized Si, respectively, using Rutherford backscattering spectrometry. To demonstrate the successful activation and quantum control of near-surface implanted Bi, the full hyperfine spectrum of these donors is obtained using continuous-wave electron spin resonance at 25 K, supporting the suitability for Bi donor qubits.

Figure 1

An example of a proposed donor qubit device (not to scale) with implanted Bi donors (red). Donors are located $\sim 20$ nm below the Si-oxide interface enabling spin manipulation by an on-chip microwave antenna and readout by coupling to a single-electron transistor (SET). The inset shows the depth profile of implanted 26 keV Bi donors simulated using srim. Control and readout of a single Bi donor could be achieved by tuning the potential of surface gates [1].

Figure 2

1 MeV ${\mathrm{He}}^{+}$ RBS spectra collected at a scattering angle of ${110}^{\circ }$. (a) The damage to the Si crystal caused by the various implants in Table 1 before annealing can be seen in the Si edge height in the channeled orientation. (The legend is in order of decreasing Si edge height.) (b) Sample D annealed at $700{}^{\circ }\mathrm{C}$ for 5 min. With the sample aligned in the channeled orientation, the Si surface peak and lower energy peak corresponding to channeling oscillation [36] are visible. The reduced area of the Bi peak for the channeled sample orientation is due to the high substitutional fraction.

Figure 3

Electrical activation yield of Bi implanted Si given (a) a 5 min RTA at various temperatures and (b) a $1000{}^{\circ }\mathrm{C}$ RTA for various times. Errors are dominated by the uncertainty in the implant fluence.

Figure 4

(a) The substitutional fraction of Bi measured using RBS at a scattering angle of ${170}^{\circ }$ (red) and the diffusion length of Bi measured from the FWHM of the Bi peak in RBS spectra collected at a scattering angle of ${110}^{\circ }$ (blue) as a function of anneal temperature for a 5 min RTA. (b) The same as a function of anneal time for a $1000{}^{\circ }\mathrm{C}$ RTA. The diffusion length data were fit with Eqs. (7) and (5) with a fixed activation energy $E_A=3.85$ eV [39], resulting in diffusion coefficients of $D_0=4.0\pm 0.5$ and $7.5\pm 0.5{\mathrm{cm}}^2{\mathrm{s}}^{-1}$ in samples without and with preamorphization, respectively. Error bars are estimated from the RBS spectra (red) and the standard deviation of Gaussian fitting to the depth profiles (blue).

Figure 5

(a) Simulated energy levels of Bi in Si as a function of magnetic field [43]. The red lines show the magnetic field at which microwaves with frequency 9.7209 GHz are on resonance with an ESR transition. (b) The full hyperfine spectrum of near-surface implanted Bi in Si measured using cw-ESR at $T=25$ K with a microwave frequency of 9.7209 GHz at 0.4743 mW. The 10 ESR peaks are labeled with their projected nuclear spin, $m_s$. The inset shows an expanded view of the $m_s=+9/2$ transition.