Influence of the sputtering parameters on the properties of Al2O3 and AlN insulators in spin tunneling junctions

The role of the sputtering parameters on the properties of thin Al2O3 and AlN insulator layers for spin dependent tunneling junctions was studied. Al2O3 insulators were prepared by rf sputtering from an Al2O3 target in an Ar or Ar-O-2 atmosphere and also by plasma oxidation in an O-2 atmosphere of a thin Al layer. The thin Al layer was deposited by rf sputtering at room temperature. AlN was prepared by reactive sputtering from an Al target in a Ar-N-2 atmosphere. The index of refraction n measured by ellipsometry, was used to optimize the sputtering parameters, and chemical etch rates were used for information on pin-hole density. The aimed values were n=1.75 for Al2O3 (corundum structure) and n=2.2 for AlN. For reactively sputtered Al2O3, under optimized deposition conditions, n reaches 1.65 for films 300 Angstrom thick. Al2O3 thin layers fabricated by plasma oxidation of a thin Al film were also prepared but the oxide layer was too thin for ellipsometry characterization. AlN insulating films show a constant n value of 1.92 from 400 Angstrom down to 60 Angstrom thick films. Rutherford backscattering analysis show that the ratio of Ar to N-2 must be less than 1 to avoid O-2 incorporation in the film. Junctions of the type glass/buffer/FM/insulator/FM, where FM is either Co or NiFe, were made by contact metal masks with a junction area of 0.25 mm(2) The Al2O3 deposited by sputtering from an Al2O3 target under conditions expected to give n>1.5 had a magnetoresistance at room temperature MR(RT) of 2.5 % with a junction resistance R-j of 2.2 M Omega and by fitting the I-V curve to existing tunneling theory gave an effective barrier thickness t(j) of 20 Angstrom and a barrier height Phi of 2.2 eV. These occurred in only one sample (9 junctions) and were not reproducible. The Al2O3 insulators prepared by plasma oxidation of a thin Al layer gave a MR(RT) = 4%, t(j) = 22 HL, R-j = 250 k Omega, and Phi = 1.4 eV. The AlN insulator with n =1.9 gave a MR(RT) = 1.6 %, t(j) = 20 Angstrom R, = 4.8 k Omega and barrier height 1.2 eV. In the last two cases junctions with MR at room temperature can be produced with reasonable reproducibility, Al2O3 samples showed a 15-20 % increase in MR as the temperature was lowered to 17 K, while AlN junctions show an increase of a factor of 2 in MR at 77 K.