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Electron-Spin-Resonance and Infrared Studies of Semiconducting, Rare-Earth-Doped CdF2

P. Eisenberger and P. S. Pershan*

  • Division of Engineering and Applied Physics, Harvard University, Cambridge, Massachusetts

  • *Alfred P. Sloan Foundation Fellow.
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Phys. Rev. 167, 292 – Published 10 March, 1968

DOI: https://doi.org/10.1103/PhysRev.167.292

Abstract

Normally insulating and transparent low-doped CdF2 becomes semiconducting and colored when baked in a Cd vapor. The electron-spin-resonance (ESR) spectrum of the semiconducting samples are compared with the insulating ones for numerous dopants. The differences found are shown to arise either from the paramagnetic "conduction" electrons themselves or from the interactions of those electrons with paramagnetic dopants. Calculations are made which explain the observed g value of 1.955 and the line-shape variations of the "conduction"-electron resonance. The properties of the observed signal in semiconducting gadolinium-doped samples are explained on the basis of a model in which the conduction electrons are coupled to the Gd3+ by an exchange interaction. The method of moments was used to calculate a theoretical spectrum for that system. The results are in agreement with the observed frequency of the exchange-coupled system as well as its angular dependence, which arises from the crystal-field interaction with the gadolinium. Optical studies on many differently doped samples from 50 000 to 30 cm−1 successfully obtain the properties of the absorption band which appears upon conversion to the semiconducting state. A broad band appears whose peak is at 0.16 eV, which agrees with the activation energy for conductivity obtained by Weller from Hall studies. However, broad wings extended into the far infrared (below 30 cm−1) as well as into the ultraviolet (for 0.1% doped samples). The use of the system as a photodetector is investigated. It is found that it has potential device applications as a detector in the infrared. Initial experiments are made to specify the physical principles of its operation as well as to determine the condition for optimum performance. All the phenomena described above indicate, in different ways, the existence of localized regions in the crystal in which the "conduction" electrons have a higher mobility than the bulk mobility of the sample, and therefore such regions play a central role in the physical model developed for the system.

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Phys. Rev. 167, 292– Published 10 March, 1968
  • Received 31 August 1967

© 1968 American Physical Society

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