Date of Award

Summer 1990

Degree Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Physics

First Advisor

Pedrotti, Frank L.

Second Advisor

Day, A.

Third Advisor

Tani, Smio

Abstract

Electrical measurements were made using the van der Pauw Hall effect technique on various III-V semiconductor substrates doped with the rare earth, ytterbium, by ion implantation. The substrates included semi-insulting (SI) and vapor phase epitaxial (VPE ) gallium arsenide, semi-insulating indium phosphide, and metallo - organic chemical vapor deposited (MOCVD) AlGaAs (AlxGal_xAs) with two compositions (x=O.3) and x=O.23) of AI. In all cases, the Yb ions were implanted into the substrate with an energy of 1 MeV to a dose of 3x10l3 cm-2. Room temperature measurements were made on all samples, and temperature dependent measurements were made on one of the AIGaAs: Yb samples down to near liquid helium temperatures. These measurements were analyzed by comparison with theoretical equations to determine the impurity ionization energy level as well as donor and acceptor concentrations by a best-fit calculation technique. It was not always possible to make definitive characterization of these samples due to their high resistivity or inhomogeneity which renders the van der Pauw technique ambiguous. Nor were sufficient samples available to allow generalizations from successful measurements. It was found that Yb produced acceptor-like activity in the SI GaAs substrate, even causing a change over from residual n-type activity to p-type activity. The VPE GaAs substrate was found to be too highly doped in itself to allow determination of the electrical effect of the implanted Yb ions. The InP substrate and implanted sample were too high in resistivity to claim a definite electrical effect. The A1GaAs substrate (x=O.3) was highly conducting, due to residual impurities that were acceptor-like. The effect of Yb in these samples reduced slightly the concentration of holes, implying a donor-like effect. However, the substrate with x=O.23 showed strong p-type activity, similar to that found in the SI GaAs samples, giving support for an acceptor-like effect. The effect of Yb in both GaAs and A1GaAs is tentatively believed to be acceptor-like, due to a predominance of interstitial positions occupied by the Yb after anneal. Such activity is consistent with a +3 valence for the Yb ions. Temperature-dependent resistivity measurements lead to an ionization energy of the controlling impurity in A1GaAs :Yb of 19.8 meV, while corresponding carrier concentration behavior gives an energy of 17.35 meV. At the lowest temperature, the latter measurements suggest a second ionization energy of 8.9 meV, due to a more shallow acceptor. These levels have been tentatively assigned to the residual acceptor in the substrate and the Yb dopant, respectively. By minimizing disagreement with theoretical equations describing single level impurities, allowing for excited states of the impurity and compensating impurities, it was determined that a best fit required the following values for the A1GaAs : Yb sample investigated: Acceptor concentration = 4.42x1013 cm-2 ; Donor concentration = 1.76x1012 cm-2. These values were determined for the intermediate temperature range, where the controlling acceptor ground state energy is 17.35 meV . These values are also consistent with a best value of 0.56 for the hole effective mass, 4 for the degeneracy factor, and zero excited states of the impurity.

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