5.4 Collection efficiency of the fluorescence light
5.4.2 SiV centers in diamond nanocrystals
Another possibility to improve the collection efficiency of the fluorescence is to avoid the refraction. If color centers are contained in diamond nanocrys- tals, the subwavelength size of the nanocrystals renders refraction irrelevant. Thus those color centers can be considered as light sources emitting in air. However, the improvement of the collection solid angle is accompanied by a reduction of the radiative transition rate from the excited to the ground state. This effect is caused by the change of the refractive index of the surround- ing medium from bulk diamond to air. In a simple approach the radiative lifetime of an optical dipole is expressed as
τn =nτv, (5.9)
where τv is the radiative lifetime in vacuum and τn is that in the material
with a refractive index of n [109].
The investigated nanocrystals were grown on a fused silica substrate using the CVD method by James Rabeau of the University of Melbourne, Australia. A picture of the sample recorded by atomic force microscopy is presented in figure 5.23. The lateral size of the diamond nanocrystals varies from 100 nm to 1000 nm, and height from 10 nm to 120 nm.
Although in bulk CVD diamond the concentration of SiV centers had been found to be so high that the optical addressing of single centers is impossible, it is reasonable to assume that the CVD diamond nanocrystals may contain single SiV centers, as the size of those nanocrystals is significantly smaller than the diffraction-limited spot of the confocal microscope. Figure 5.24(a) shows a 20x20 µm2 scan on the sample. Strong and localized fluorescence
82 Single SiV centers
Figure 5.24: (a) 2D scan over an area of 10x10 µm2 on the sample with
diamond nanocrystal. (b) A point-like emitter found at the position marked by the white square in (a).
from SiV centers was observed, which corresponds to the large crystals with a high content of SiV centers. In a dark region which is marked by the white square on this scan a point-like emitter was found as presented in figure 5.24(b).
The spectrum of this point-like emitter is presented in figure 5.25 and the ZPL of the SiV center is clearly visible. The Raman spectrum of diamond disappears due to the small size of the crystal. Despite the high brightness of this emitter, the second-order correlation function shown in figure 5.26 ex-
Figure 5.25: Spectrum of the fluorescence light of the point-like emitter pre- sented in figure 5.24
5.4 Collection efficiency of the fluorescence light 83
Figure 5.26: Second-order correlation function of the fluorescence light of a single SiV center in a CVD diamond nanocrystal.
hibits a strong photon antibunching effect, which indicates that the presence of only a single SiV center in the nanocrystal. Differently from the bright single SiV centers observed with SIL, almost no effect of correlated emission effect is present in g(2)(τ), which suggests a high position of the Fermi-level
in the diamond nanocrystals. The fit parameters were determined as follows: c=−1.13±0.03, τ1 = 1.35±0.21 ns and τ2 = 84.1±55 ns.
Compared with the single SiV centers in undoped bulk samples with the same excitation power, the photon count rate of this SiV center is higher by a factor of 11, although according to eqns (5.5) and (5.9) only an improvement of 2.8 is expected due to the small size of the nanocrystal. On the other hand, the second-order correlation function reveals a change in the emission characteristics of the SiV center. The results of the least square fit of g(2)(τ)
are similar to those of the brightest SiV center in the samples with nitrogen doping. Thus the Fermi-level in the nanocrystal is probably close to the conduction band so that the possibility to populate the positively charged state is negligible. This is possible since nitrogen is a dominant impurity in CVD diamond, and the size of the nanocrystals ensures a short distance be- tween SiV centers and nitrogen impurities. Another important information contained in the second-order correlation function is that the short time con- stantτ1 remains unchanged in nanocrystals. τ1 is considered to be primarily
determined by the transition rate from the excited to the ground state. Ac- cording to eqn (5.9) the radiative lifetime should be enlarged by ndiam in
nanocrystals. This observation confirms that the transition from the excited to the ground state of the SiV center is dominated by non-radiative channels.
Chapter 6
Other color centers
There is a surprising variety of color center species in diamond. Due to the low quantum yield at room temperature, the SiV center is not ideal for single photon generation in any practical applications. Therefore, the possibility of using other color centers was explored. For this purpose photoluminescence investigations of xenon and nickel implanted diamond samples have been carried out.
Furthermore, series of single color centers were observed in IIa diamond during the extensive scanning, although due to the limitation of the setup only the fluorescence in the spectral range from 730 nm to 760 nm is visible. Because of their low concentration such color centers are hidden in macro- scopical luminescence investigation of diamond, therefore no report of them can be found in literature. The determination of the origin of those color centers is beyond the scope of this thesis. Here, we merely document the observed single color centers.
6.1
Xenon implantation
Xenon ion implantation has been widely used to study ion-beam-induced transformation of isolating diamond to a conductive form of carbon [114, 115]. Photonluminescence spectra of xenon implanted natural Ia diamond exhibit a single ZPL at 811.6 nm at a temperature of 8 K. With increasing temperature a second ZPL at 793.3 nm appears and grows, while the ZPL at 811.6 nm becomes less intense. This behavior indicates that these two ZPLs originate from two sub-levels of the same excited state. A spectrum of the Xe-related center at 200 K is presented in figure 6.1. The Huang-Rhys factor was determined to be S = 0.69±0.14, which reflects a moderately low electron-phonon coupling [116]. Thus the Xe-related center could be
86 Other color centers
Figure 6.1: Spectrum of the Xe-related center at 200 K [116]. interesting for single photon generation.
The investigated samples were two natural IIa diamonds implanted by 2 MeV Xe ions at room temperature with a dose of 1013 cm−2 (sample Xe1)
and 1011cm−2 (sample Xe2). Post-implantation annealing was carried out at
1400 ◦C in vacuum for 1 hour. For the photoluminescence investigation the
confocal microscope was modified. Basically the dichroic mirror DICH690 was replaced and an interference filter IF810 was inserted in front of the APDs. The transmission curves of the dichroic mirror and IF810 are shown in figure 6.2. For the excitation, the 690 nm laser diode was used.
A scan along the optical axis of the implanted surface of the sample Xe1 was recorded and is shown in figure 6.3(a). The sharp peak corresponds to
Figure 6.2: Transmission of the dichroic mirror (red line) and the interference filter IF810 (green line) used for analysis of the Xe-related center.
6.1 Xenon implantation 87
Figure 6.3: (a) Scan along the optical axis on the implanted surface of the sample Xe1. (b) Spectrum of the fluorescence.
very intense fluorescence from the diamond surface. In order to avoid satu- ration of the APDs, a neutral density filter with a transmission of 0.04 was set into the optical path to attenuate the fluorescence light. The spectrum of the fluorescence is shown in figure 6.3(b). The characteristic two ZPLs of the Xe-related center are clearly visible. While the ZPL with higher energy is found at 793.7 nm, which agrees well with the value in [116], the ZPL at 814.3 nm shows slight deviation from the literature value. Homogeneous dis- tribution of the fluorescence light intensity was found on the surface, which indicates a high concentration of the Xe-related centers.
The intensity of the fluorescence light from the implanted surface of the sample Xe2 is much lower. Figure 6.4(a) shows a longitudinal scan of the surface recorded without the neutral density filter. The background emis- sion from the diamond sample is not negligible compared to the peak which corresponds to the fluorescence of the Xe-related centers. The spectra of the
Figure 6.4: (a) Scan along the optical axis on the implanted surface of the sample Xe2. (b) Spectrum of the fluorescence (red line) and background emission of the diamond sample (blackline).
88 Other color centers
Figure 6.5: (a) 2D scan on the implanted surface of the sample Xe2, recorded with a integration time of 3 s for each point. (b) Histogram of the photon count rate for the area shown in (a).
fluorescence and the background emission are shown in 6.4(b). The back- ground emission originates primarily from the second-order Raman scatter- ing of the diamond lattice which is much broader than the first-order Raman line [117]. In order to compare the concentration of the Xe-related centers in the two samples, the intensities of the ZPL at 814.3 nm in the spectra were normalized by the first-order Raman line. We found out that the sample Xe1 has a higher concentration of Xe-related centers than the sample Xe2 by a factor of 21.
The results presented above demostrate the fabrication of Xe-related cen- ters by ion implantation, although the concentration of Xe-related centers is too high to detect single ones. Before preparing samples with lower xenon ion dose, it is reasonable to consider the photon emission rate expected for a single Xe-related center. The answer to this question requires a quantitative estimation of the concentration of the Xe-related centers in the samples. The experimental measurable value is the photoluminescence intensity instead of the concentration of the color centers in the sample. As the ion range in the direction of the optical axis is significantly smaller than the optical res- olution, the spatial concentration of the Xe-related centers is equivalent to their areal density on the surface which is proportional to the photon count rate recorded by the confocal microscope. On the other hand, according to the conditions of the ion implantation we assume a homogeneous distri-