Programmable quantum emitter formation in silicon

Abstract

Silicon-based quantum emitters are candidates for large-scale qubit integration due to their single-photon emission properties and potential for spin-photon interfaces with long spin coherence times. Here, we demonstrate local writing and erasing of selected light-emitting defects using femtosecond laser pulses in combination with hydrogen-based defect activation and passivation at a single center level. By choosing forming gas (N₂/H₂) during thermal annealing of carbon-implanted silicon, we can select the formation of a series of hydrogen and carbon-related quantum emitters, including T and C_i centers while passivating the more common G-centers. The C_i center is a telecom S-band emitter with promising optical and spin properties that consists of a single interstitial carbon atom in the silicon lattice. Density functional theory calculations show that the C_i center brightness is enhanced by several orders of magnitude in the presence of hydrogen. Fs-laser pulses locally affect the passivation or activation of quantum emitters with hydrogen for programmable formation of selected quantum emitters.

Fig. 1. Programmable quantum emitter formation with femtosecond (fs) laser pulses in silicon-on-insulator (SOI).

Artistic representation of the fs laser irradiation approach to locally write and erase G, and C_i centers in SOI. The G center is a pair of two carbon atoms at substitutional sites (black sphere) combined with the same Si self-interstitial (pink sphere), whereas the C_i consists of a pair of an interstitial carbon (green sphere) and a substitutional Si atom (gray sphere) in the Si lattice. A single fs laser pulse (pulse duration of 90 fs), with wavelength centered at 800 nm, was used for irradiation at varied fluences to locally form and erase quantum emitters. Three different areas are highlighted to represent the workflow starting with a pre-processed SOI wafer with ensemble C_i centers formed after ion implantation and rapid thermal annealing under forming ambiance (area on the left). The second area (in the middle) represents the writing of G centers along with modified C_i centers on the pre-processed SOI sample via single fs laser pulse irradiation at relatively low fluences (<30 mJ/cm²). The third area (on the right) shows the erasing of quantum emitters after irradiation with a relatively higher laser fluence (still much below the melting threshold for Si). A photoluminescence hyperspectral scan with fs laser pulse irradiation spots processed at a fluence of 12 mJ/cm² is shown in the top-right corner.

Fig. 2. Writing and erasing of G and C_i center with fs laser pulses below the damage threshold of Si.

a Process flow represented by measuring the PL spectra starting from the pre-processed SOI sample (emission ~1452 nm, corresponding to the C_i center after carbon ion implantation (7e13 C/cm² fluence) followed by rapid thermal annealing at 800 °C for 120 s under forming gas ambiance). The last three curves show the PL spectra obtained after fs irradiation at different laser fluences in order to first write G centers along with modifying the density of pre-existing C_i centers (~16 mJ/cm²), followed by partial and complete passivation of G and C_i centers, respectively, at higher fluences (~30 mJ/cm²). The final curve presents the reoccurrence of G and C_i centers after irradiation with an even higher fluence pulse (i.e., 44.5 mJ/cm²). The damage threshold in our experiments was >100 mJ/cm². b PL emission peak intensity of G and C_i centers after irradiation with fs pulses of varying energies. c TR-PL signal from the C_i centers before and after the fs laser irradiation to extract optical lifetimes. d Optical lifetimes as a function of fs laser fluence for both G and C_i centers extracted by fitting the TR-PL signal with a first-order decay function. Error bars shown in (b–d) are statistical errors from sample to sample that map to variations in the pulse energy of the fs laser output observed in our experiments. Pink and green color-band in (b) and (d) are representative of the likelihood of the absence of G and C_i centers, respectively.

Fig. 3. Isolated of C_i and G center formation post fs pulse irradiation.

a PL spectrum (recorded with 600 g/mm grating with a resolution of 0.05 nm) corresponding to the single C_i (top row) and G (bottom row) centers along with a typical green laser excitation power-dependent PL (insets). b Polarization sensitivity of the PL emission from C_i and G center (top and bottom row, respectively) along with ensemble background mapping to the single center emission. c Optical lifetime measurement for isolated C_i and G center (top and bottom row, respectively) along ensemble background. (Significantly longer optical lifetimes were observed for the isolated G and C_i centers in comparison to their ensemble counterpart). d Background corrected second-order auto-correlation signal with phenomenological fit for both C_i and G centers (top and bottom row, respectively) using Hanbury-Brown and Twiss (HBT) interferometer. The experimental value of the function at zero time delay was observed to be much below 0.5, proving the single photon emission from both centers. All the measurements on an irradiation spot with fs pulse fluence of 8 mJ/cm².

Fig. 4. Defect levels, structures, and modifications of the C_i center.

Structure of a the C_i center, b the displaced “B” configuration of the C_i center, and modified versions of the C_i center with H bonded to either c the carbon, or d the Si atom. Atoms are colored as follows—Si (gray), carbon (black, green), H (red), Si self-interstitial (pink). e Energy level diagrams for the neutral charge state of the C_i center, with conduction band (green), valence band (blue), localized defect levels (red), and electron occupation (black arrows) indicated. Panels on the right show the real space wavefunctions corresponding to each localized defect level.