7.3.1 UV-VIS Absorbtion Spectroscopy
Intrinsic, non irradiated scCVD diamonds are fully transparent in the VIS - UV range revealing an edge absorption at 224 nm, which corresponds to the electrons transition from the VB maximum to the CB minimum. Heavy irradiation induces a variety of lattice defects, depending on the radiation type and the type of diamond itself. The localized energy states introduced within the diamond band-gap lead to a modification of the intrinsic absorbtion spectra. The most comprehensive review of optically active centers in diamond can be found in [Zai01].
Figure 7.9 shows UV - VIS absorbtion spectra measured at RT with sample BDS13 before irradiation (black line), as well as the spectra for four diamonds irradiated with neutrons and protons (colour lines). Clear deviation from the ’edge’ absorption is observed for all irradiated samples indicating the bulk damage. Two separate, irradiation induced broad absorption bands can be distinguished: the General Radiation (GR1) phonon side band (1.6 eV - 2.3 eV), and an ’ultraviolet continuum’ band (2.8 eV - 5.5 eV), respectively.
The latter results from charge-carrier photo-excitation from defect ground states to the CB or/and to the VB. While the shape of the spectra are similar for all samples, a spectral identification of defects at RT is difficult due to phonon washout. At low temperature where lattice vibrations are ’frozen out’, zero-phonon transitions are possible, giving rise to a sharp spectral line(s) specific for a given defect.
In order to detect Zero-Phonon Lines (ZPL) of radiation-induced defects, UV - VIS absorbtion spectra were measured with BDS13 at cryogenic temperatures ranging from 7 K to 77 K. Figure 7.10 shows the spectrum obtained at 7 K.
Three ZPL are observed:
7.3. Radiation-Induced Defects Identification 107 john100 after 2x1015 20MeV n/cm2 BDS13 after 1.18 x 1016 26MeV p/cm2
Figure 7.9: UV - VIS absorbtion spectra of neutron and proton irra-diated diamonds, measured at RT.
Two continuous bands appear after hadron irradiation: the GR1 band, related to the mono-vacancy, and a band referred as the ultraviolet-continuum, the result of a charge carriers photoexcitation from defect states to the VB or/and to the CB.
2 3 4 5
Figure 7.10: (Left panel) UV - VIS absorbtion spectrum of BDS13 measured at 7 K after 26 MeV proton irradiation . Three ZPL can be distinguished, related to GR1, R2 and R11 centres. (Right panel) Detailed view of the absorbtion spectrum around ∼1.673 eV, showing the ZPL of the GR1 center.
• at 1.673 eV (740 nm); the ZPL of the GR1 center, which is attributed to a neutral mono-vacancy defect V0 [Dav02, Lan68];
• at 1.859 eV; the ZPL of the R2 center, which is attributed to a neutral isolated
< 001 >-split self interstitial defect I<0001> [New02];
• at 3.98 eV; the ZPL of the R11 center, which is produced by a transition to an excited state of the R2 center [All98].
The absence of the ND1 at 3.15 eV and other nitrogen related centers like H3 at 2.463 eV, is a proof of the low concentration of nitrogen impurities in the investigated scCVD dia-monds. Furthermore, the absence of ZPL like 5RL (cluster of interstitials) at 4.581 eV, R1
The concentration of neutral mono-vacancies N [V0] and self interstitials N [I<0001>] can be calculated from the integral intensities of the GR1-ZPL and the R2-ZPL respectively, according to the following equation:
N [V0] = Astr
f (7.9)
where Astr is the absorption strength of the ZPL of GR1- or R2 centers given by:
Astr =
ZP L
α(E)dE (7.10)
and f is the calibration constant of proportionality between the absolute defects concentra-tion and the ZPL intensity obtained from ESR measurements by [Twi99]. The calibraconcentra-tion constants for GR1- and R2-ZPL amount to fGR1 = 1.233 × 10−16 meVcm2 and fR2 =1 × 10−17 meVcm2, respectively.
From Equation 7.9 integrating the ZPL of GR1 (Figure 7.10) (Right panel) one gets the concentration of the produced vacancies for sample BDS13 N [V0] ≈ 1.6 × 1017 cm−3. A similar integration of the R2-ZPL center gives N [I<0001>] ≈ 3.8 × 1014 cm−3. Only a small part of single interstitials survive after irradiation at RT, compared to the neutral mono-vacancy concentration. The vacancy-production rate is estimated to ∼13.5 cm−1. The obtained value is about∼20 times lower than the value calculated from the simulated N IEL proton curve, indicating that due to self-annealing processes only, about 4 % of the primary created defects survive.
Assuming a constant damage rate increasing proportional with the proton fluence, V0 concentrations are estimated for the rest of the irradiated samples: BDS14 - N [V0]≈ 8.65
× 1014cm−3, EBS3 - N [V0]≈ 8.24 × 1015cm−3and s256-05-06 - N [V0]≈ 1.44 × 1015cm−3.
7.3.2 Photoluminescence Spectroscopy
Set-up and Methodology Micro-photoluminescence characterization of the four neu-tron irradiated diamonds were carried out at a temperature of 77 K using a LabRam HR800 equipment, where the source of excitation is the 514.0 nm Argon-ion laser. The laser beam was focused in a spot of about 10 µm and the PL light was collected by a confocal mi-croscope, where the focal point was adjusted about 20 µm below the diamond surface. In order to probe the homogeneity of the damage, a few spots were chosen for each sample in regions close to the edge and close to the middle of the samples.
Results and discussion PL spectra of all samples are plotted in Figure 7.11. The sharp line furthest to the left corresponds to the wavelength of the excitation light, the next line to the first order anti-Stokes Raman shift of the excitation light with a Raman frequency of 1332 cm−1, which is the fingerprint of the diamond structure. A further strong luminescence peak is observed at 742 nm, which is the ZPL of the GR1 center, related to the excited state of the neutral mono-vacancy.
The broad background might be attributed to the surface defects arising from polishing processes - this hypothesis needs further verification. The zoomed spectra of two samples
7.3. Radiation-Induced Defects Identification 109
Figure 7.11: Photoluminescence (PL) spectra of neutron irradiated diamonds, measured at 77 K. One radiation induced strong PL center has been found at 741 nm (1.673 eV), which corresonds to the ZPL of the GR1 center.
(s014-09 and BDS12) are plotted in Figure 7.12. The logarithmic scale is used to demon-strate presence of the residual defects. A few sharp ZPLs of low intensity were found: the neutral nitrogen mono-vacancy complex (N V0) at 575 nm, and the neutral <0 0 1>-split self-interstitial (I<0001>) at 664.5 nm. The origin of the other ZPLs is not known. However, comparing the intensity of those residual lines to the GR1 intensity, for example for the BDS12 sample (lowest irradiation fluence), one can see that the peak area ratio is in order of 0.001. In conclusion, as it was also the case for the 26 MeV proton irradiation, the neutral mono-vacancy is the main defect produced by the neutron irradiation.
Although photoluminescence is a very sensitive technique, it cannot give absolute num-ber of defect concentrations. Nevertheless, by normalizing the spectra to the intensity of the first Raman line, a relative comparison of the defect concentration can be made for the four irradiated samples. Figure 7.13 (Left panel) shows the region around 741 nm with the GR1 peaks of all irradiated samples. The area below the peaks is proportional to the concentration of vacancies. The 2.4 meV split of the GR1 ground level state observed for the sample john100, is most probably induced by stress in the diamond lattice [Bra81].
The average values of GR1 integrals are plotted in Figure 7.13 (Right panel) as a function of the neutron fluence. Perfect linearity is found indicating a constant production rate of mono-vacancies for the range of the integral fluences applied.
Figure 7.12: Zoomed PL spectra of two scCVD revealing residual defects. Among them, the complex of nitrogen and the neutral mono-vacancy N V0 at 575 nm and a <0 0 1>-split interstitial I<0001> at 664.5 nm may be identified. Comparing to the GR1 intensity, the residual defects concentration is negligible.
739 740 741 742 743 744
0.0 0.1 0.2 0.3
BDS12 s014-06 s014-09 john100
normalized PL [a.u]
wavelenght [nm]
741.2 nm
∆E=2.4 meV
0 1x1015 2x1015
0.00 0.05 0.10 0.15 0.20 0.25 0.30
normalized GR1 intensity [a.u.]
fluence [20MeV n/cm2]
Figure 7.13: (Left panel) Normalized PL signals at 741.2 nm (ZPL of the GR1 center) from neutron irradiated scCVD diamonds. The split of the line observed from sample john100 indicates presence of strong lattice stress [Bra81] (Right panel) Intensity of the GR1 line as a function of the integral neutron fluence.