bution for the xenon ions in the sample and consequently for the created Xe-related centers. Thus, the concentration of Xe-related centers exhibits a spatial fluctuation of √ρ, where ¯¯ ρ is the average concentration. Therefore, we can determine the average concentration of Xe-related centers by measur- ing the spatial fluctuation of their fluorescence intensity over the implanted diamond surface.
Figure 6.5(a) shows a 2D scan over a 20x20µm2 of the implanted surface
of the sample Xe2 recorded at the maximal excitation power. During scan- ning several sources of the photon count rate fluctuation were considered, including statistical errors in the photon counting, the stability of the ex- citation power and the position of the surface, and their contributions were found to be negligible. A histogram of the photon count rate for this area was measured and is presented in figure 6.5(b). The solid line in figure 6.5(b) is a least square fit using a Gaussian function. The fit function suggests an aver- age value of ¯c= 6876±7.2 s−1and a standard deviation of ∆c= 184±7.2 s−1
for the photon count rate. The longitudinal scan shown in figure 6.4(a) which was recorded at the same excitation power suggests a background count rate ofb= 1500±50s−1, including emission from the diamond and dark counts of
the APDs. From these data an average number of Xe-related centers within the focus of the confocal microscope was estimated as
¯ c−b ∆c 2 = 851±34. (6.1)
The focus has a FWHM of about 0.7 µm and the areal density of the xenon atoms is 1000 µm−2. Therefore, the possibility that a xenon atom forms a
Xe-related center is nearly unity. The average photon count rate of a single Xe-related center was determined to be about 7 s−1.
Several reasons are responsible for this extremely low value. First of all, only a part of the fluorescence light was detected due to the interference filter IF810. Secondly, the excitation might not reach the saturation power. Finally, the focus of the confocal microscope has a Gaussian profile. If a single Xe-center is located at the middle of the focus, it will exhibit higher photon emission rate. Nevertheless, the Xe-related center is not an efficient source for single photon generation. Therefore, no further investigation of this color center was carried out.
6.2
Nickel implantation
Nickel impurities are often present in HPHT (High Pressure High Tempera- ture) diamond, since nickel is usually used as catalyst for diamond synthesis.
90 Other color centers
Figure 6.6: Spectra of the nickel implanted Ia diamond samples and annealed at different temperature.
For many years, the only experimental evidence of the presence of the heavy metal element in diamond was the ESR (Electron Spin Resonance) spectrum of nickel which originates from the hyperfine structure of 61Ni [118]. Nickel
exists not only as single disperse atoms in diamond. It can also form a vari- ety of complexes with nitrogen atoms at high annealing temperatures. The nickel-nitrogen defects with different configurations were identified by ESR experiments and labeled as NE1-NE5. Nadolinny et al. reported on a new line in the ESR spectra of 13C enriched diamond and denoted it as NE8.
The structure of the NE8 center is considered to consist of one nickel atom surrounded by four nitrogen atoms. Photoluminescence investigations of the samples revealed a ZPL at 793.6 nm which is stable up to 2300 K just like the ESR line of NE8, thus this ZPL was ascribed to the optical transition of NE8 [119]. The Huang-Rhys factor of the 793.6 nm center is determined as S = 3.5 [120, p. 149].
Recently the observation of single color centers with sharp ZPLs around 800 nm has been reported by several groups. Gaebel et al. reported on a color center in untreated natural IIa diamond. It has a sharp ZPL at 802 nm and a luminescence lifetime of 11.5 ns [26]. Wu et al. found also another color center in natural IIa diamond with a ZPL at 782 nm and a luminescence lifetime of about 2.1 ns [29]. Rabeau et al. presented a color center incorporated in CVD diamond nanocrystals. The ZPL of that center is at 797.6 nm and the Huang-Ryhs factor is determined to be S = 0.35 [27]. Although the color centers have different optical properties, they are claimed to be the NE8 center. As there are many unknown color centers present in diamond in vanishing concentration (see §6.3), those reported single color centers might also have different origins.
6.2 Nickel implantation 91
Figure 6.7: Spectra of the nickel implanted IIa diamond samples and annealed at different temperature.
diamond sample, nickel ion implantation of diamond was carried out. As host materials both Ia and IIa diamonds were chosen. Ia diamond contains nitrogen impurities in aggregate form which could be advantageous for the formation of nickel-nitrogen centers. Three Ia diamond samples were im- planted by 2 MeV nickel ions with a dose of 1013 cm−2. Post-implantation
was carried out in vacuum for half an hour at 1000◦C, 1200◦C and 1400◦C,
respectively. Two IIa diamond samples were implanted at similar conditions, and annealed at 1200 ◦C and 1400 ◦C.
The photoluminescence spectra of the implanted surface of the Ia di- amond samples are shown in figure 6.6. The fluorescence light probably originates from the neutral vacancies produced by ion implantation. The neutral vacancy is responsible for the GR1 band with a ZPL at 740 nm. At room temperature the ZPL is weak compared with the vibronic sidebands, and it is as well suppressed by the dichroic mirror in the confocal micro- scope. Therefore, it is not visible in the spectrum. The intensity of the GR1 band, normalized by the Raman line, decreases with increasing annealing temperature.
The photoluminescence spectra recorded from IIa diamond samples are presented in figure 6.7. The sample annealed at 1200 ◦C exhibits the same
spectrum as the Ia diamonds, although with much lower intensity, while the GR1 band almost completely disappears in the sample annealed at 1400◦C.
Generally, the dependence of the results of the annealing process on the temperature can be understood by analyzing the mobility of vacancies. The vacancies in IIa diamond seem to have a higher mobility than in Ia diamond, because the IIa diamond samples show a lower intensity of the GR1 band than the Ia diamond samples after same implantation and annealing process. The reason of the different mobility of vacancies in the two types of diamond
92 Other color centers
is unknown. It might depend on the content of impurities in the samples. No sharp ZPL around 800 nm was observed in both Ia and IIa diamond, although the concentration of nickel impurities in the implanted region is considerably high. This indicates that due to the complex structure the NE8 center is much more difficult to be created than the SiV or the Xe-related center. The nitrogen atoms in diamond start to diffuse at much higher temperature than vacancies. The formation of the NE8 center in synthetic diamonds with high content of nitrogen and nickel can only be observed after heating above 1600 ◦C [120, p. 149 ]. This might explain the failure of out attempts. Since
the higher annealing temperatures are associated with considerable technical complexity, no further work has been done to fabricate the nickel-nitrogen center in this work.