Electron Paramagnetic Resonance Study of Ion-Implanted Photorefractive Crystals
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369 Mat. Res. Soc. Symp. Proc. Vol. 504 ©1998 Materials Research Society
EXPERIMENTAL X-cut( the C-axis perpendicular to the ab plane) single crystals of LiNbO 3 , MgO, and A120 3, were polished by EPI technique and were implanted with 160 keV Fe÷ at room temperature at fluences of I X×14 /cm 2 and I XI 016/cm 2 . Also, LiNbO 3 crystals were implanted with 2.0 MeV Mn at room temperature at fluences of 2.5x1014 /cm 2 and 2.5 x1016/cm 2 . Electron Paramagnetic Resonance was taken using a Bruker X-band 300ESP spectrometer. The modulation frequency was 100 kHz and the microwave frequency was 9.3 GHz. The sample was placed in an optical while acquiring the spectrum. Diphenyl picryl-haydrazyl (DPPH) with a g value of 2.0036 was taken as a g-marker and a reference for the calculation of the spin concentration. A HeNe laser with 35mW power was used as light source. RESULTS AND DISCUSSION The EPR spectra for the three crystals were taken at room temperature and the laser illumination was done in situ to observe the decay and the growth of the EPR signal i.e. the charge transfer due to the laser illumination. The EPR spectra are shown in figures 1 to 5. For LiNbO 3 : Mn2+ as shown in figure 1; there is one group of sextet due to Mn2+ (s=5/2) and two strong lines due to Fe 3+. This assignment is based on the fact that the sextet-lines are due to hyperfine interaction of the d electrons of Mn with the s=5/2, "Mn nucleus. One strong set of sextets are observed, at 0 =0° and 900, the other weak two are overlapping with the Fe broad line which decreases the resolution of the hyperfine structure splitting. It is clear that the crystal contains some Fe3+ ions which is unintentionally doped but the trace of Fe 3÷ does exist on the starting materials as impurity. Fe 3+ is playing the main role in the photorefractive processes in these crystals. There are three possible sites for paramagnetic impurities in LiNbO 3, a Li site, a Nb site and a structural vacancy. Arguments have been advanced which favor both Li and Nb sites as possible candidates for Fe 3÷ impurity. EPR studies of Cr 3÷ in LiNbO 3 have shown that both Li and Nb sites are possible candidates for the Cr 3+impurity. To interpret the EPR for the LiNbO 3: Fe crystal, Fe3+ (S =5/2) was found in an axially symmetric site and is described by the spin Hamiltonian [7] H=g13H*S+B°2 002 + B0 4 004 +B34 034 In this expression 002 =35 2z - S(S+I), 004 =
35
S4z
-30S(S+1)
S2z +25 S2z -6S(S+I),
W I/ [SZ (S3+ + S3_ ) + (S3+ + S3_ ) Sz] At 0 =0, the energy eigenvalues are given by [6] 2 0 0 E(±5/2) = ±gI 1 H + B° 2 + 90 B 4 + {[±(3/2) gl fI H + 9 B 2 -30 BO4 ] +90 I B3412}1/2 0 E(±3/2) = ±3/2 gl 1 H - 2B11- 180 B 4, 0 2 0 0 0 E(±l/2) = +g 13 H + B 2 + 90 B 4 - {[+(3/2) glI3 H + 9 B 2 -30 B 4] +90 B3412}1/2
370
The crystal field parameters at room temperature were found to be 0.922 G and B 34 = l0 G
B
0 2
= 540 G,
0
B 4 - -
In the case of MgO3 the EPR, spectra are shown in figures 2, 3, 4 and 5 for low and high fluences. The hyperfine splitting for the Fe3+ is resolved even
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