Electron Beam Processing of ZnGeP 2 : A Nonlinear Optical Material for the Infrared
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EXPERIMENT Single-crystal samples of random orientation were cut from ZnGeP 2 boules grown by the horizontal gradient freeze technique. The samples are listed in Table I, which shows the melt composition of each boule along with the sample thickness and measured lpm-absorption coefficient a before irradiation. (This was chosen as a representative wavelength since it corresponds roughly to the peak of the absorption shoulder.) Note that a is the lowest for nominally stoichiometric material, increasing with excess Ge as well as excess ZnP2. Note also that the absorption coefficient for sample 13A (cut adjacent to sample 13C from the same boule) dropped by -30% upon annealing. Table I. ZnGeP 2 Samples Used for E-Beam Irradiation Study Sample No.
*
Composition
Thickness
ltm Absorption
(mol % ZnP 2)
(mm)
Coefficient (cm- 1)
liC
43.8
1.72
13.60
3A
46.8
4.15
11.05
13C
49.96
0.75
8.75
13A*
49.96
0.94
6.34
16C
51
0.76
13.72
Annealed at 500"C for 300 hours on ZnP 2 powder followed by 5000 C, 300-hour anneal in red phosphorus powder.
Electron-beam irradiation was performed using the Phillips Laboratory Dynamitron Accelerator at Hanscom AFB. Radiation was applied in four sequential doses (designated RADI, RAD2, RAD3, and RAD4) using the experimental parameters listed in Table II. (These parameters were comparable to those used by Brudnyi et at5: 2MeV, lpA/cm2 , 300K, electron flux 4, = 2.6 to 16 xl017cm 2.) The ZnGeP2 samples listed in Table I were clipped with phosphor-bronze tabs to a specially-fabricated holder which featured four jets fashioned from copper tubing through which liquid nitrogen slowly sprayed over their surface in order to prevent a large temperature rise during irradiation. This cooling fixture maintained the sample temperatures between 50C and 25"C to minimize thermal annealing of the radiation induced defects. The samples were then mounted 4 inches from the source window and irradiated for
3-4 hours to the required fluence levels, which corresponded to 1-1.5MeV dose rates from 49xl0'4 e-/cm 2/sec. RESULTS AND DISCUSSION 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 data were corrected for reflectance loss as a function of wavelength using refractive index values derived from the published sellmeier coefficients.7 The resulting
absorption spectra are 730
Table II. Electron-Beam Irradiation Experimental Parameters. RAD I Electron Fluence,
5.0x1017
(e-/cm2)
RAD 2
RAD 3
RAD 4
17
1.06xl08
7.5x10l'
9.4x10"
2.00x10"
2.75x10"
4.4x10
_
5.0xl017
Cumulative Fluence, (e-/cm2) Electron
1.02
1.5
1.0
1.0
200
400
400
400
5-25
5-20
5-15
5-20
Energy (MeV)
Analyzed Beam Current
(uA)
Sample Temp.
Table 111. Effect of Subsequent E-Beam Irradiation Doses on the 1jim Absorption Coefficent. Sample No.
RAD I
RAD 2
RAD 3
11C
13.01
12.04
11.60
3A
11.39
11.16
10.88
13C
7.47
6.62
4.76
4.28
13A
5.49
4.40
4.42
5.09
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