Gamma Ray Processing of ZnGeP 2 : A Nonlinear Optical Material for the Infrared
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of this work was to gain more insight into the nature of the absorbing native defect(s) in this material. 579 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
EXPERIMENT Single crystal samples of ZnGeP 2 were irradiated using the Phillips Laboratory cobalt-60 source at Hanscom AFB. The 40-kiloCurie source emits gamma rays with energies of 1.173 and 1.332 MeV, and consists of a circular array of 20 stainless steel tubes containing 'Co slugs located in a shielded pit. The samples were exposed to an average flux of 4.8x106 rads/hr for several months at a time to accumulate doses on the order of 101'rads. Sample temperatures during the exposure ranged between 95-100 C. Two separate sets of gamma-ray irradiation experiments were conducted. In the first set, single-crystal samples of generally random orientation were cut from ZnGeP 2 boules grown by the horizontal gradient freeze technique. The samples represented a variety of compositions, geometries (wafers and bars), and thermal histories (as-grown and annealed), as listed in Table I. Also listed in Table I are values for the li/m-absorption coefficient cL (a representative
wavelength corresponding roughly to the peak of the absorption shoulder) measured before and after two sequential doses of 2.56x10°' rads (5338hrs) and 1.35x10°' rads (3004hrs). These doses were equivalent to 2.5MeV-electron fluences of 1.02x10' e/cm2 and 5.4x10' 7 e/cm2 respectively. Table I. Absorption Coefficient Values for ZnGeP 2 at Ipm (Thin Samples) and 2.05pm (Thick Samples) Before and After Gamma-Ray Irradiation Sample No.
Composition Dimensions (mol% ZnP 2) (mm)
3E 1OB 10C lIB 13B 15F" 16F 16H*
46.8 48.3 48.3 43.8 49.96 48.5 51.0 51.0
5.2x4.5xl.27 10.0x4.5x1.61 5.5x4.9x4.29 Irreg. x 1.61 Irreg. x 1.50 Irreg. x 1.31 6.3x6.OxO.81 6.2x6.0x1.05
15E* 15K
48.5 48.5
19.2x6.4x5.0 22.9x8.3x6.8
Before Irradiation
I st Dose (2.56x10'rads)
2nd Dose (1.35x10°'rads)
Ipm Absorption Coefficient (cm-') 16.08 13.19 11.66 11.65 9.45 8.58 10.23 8.94 8.22 14.64 12.57 11.45 7.61 5.26 4.28 6.80 4.86 3.99 19.85 15.72 14.48 12.45 9.71 9.40 2.05/im Absorption Coefficient (cm-') 0.42 0.28 0.23 0.96 0.63 0.58
"Annealed (550"C, 300hrs, in ZnP 2) After initial success (described below) with the randomly-oriented material, several 6x6xl2mm3 samples were cut from high-optical quality boules with the proper crystallographic orientation for doubling the wavelength of a 2p1m-laser via "type I" optical parametric oscillation (c-axis 55" from the major axis in the plane of the two large a-axis faces). These samples (listed in Table II) were annealed (5500C, 300hrs, in ZnP 2), polished (6x6mm 2 end faces), and gammairradiated in a separate experiment with a single dose of 2x101° rads. After each radiation dose, the absorption spectra for each sample was remeasured between 0.6 and 3.2 microns using a Perkin-Elmer Lambda 9 UV/VIS/NIR dual-beam spectrophotometer. The beams were unpolarized for measurements on the randomly-oriented crystals (Table 1), but 580
Table 11. Effects of Thermal Annea
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